Hello, I'm Ler and I came in here to share the results of my researches and experiments on Master Yi with other people over the last 8 monthes (yes, I'm one of first guys who thought that THydra with Sterak on him would be a good idea. No, I cannot prove that and I don't actuallyy wanna to). First of all - you can't win by just building this stuff. I'll highlight the main aspects of mastering this master and describe things as much as possible, but the point is - to play good you need to think. To think about what item or action fits the situation best. To think both tactically and strategically. That's the key to achieving whatever you want to achieve.
As for myself, i dropped the wish to play for the win a long time ago. I play for fun and only rankeds, just cuz they're most fun. Some people don't like it - but it's just my position and my style. League is a game first of all, so why should I take it serious. But let me make it clear. I'm not a "troll" in any of the meanings, not at all. I may be not a perfect player, might be not even good (though I myself think other way), but all this was tested and not only by me. My strongest power is planning, so even if I sometimes lack reaction or interest in game process, I yet know what I'm talking about. Otherwise i wouldn't just have enough patience to write this all (especially while I'm half-sure that all was just a waste of time). Anyway though, it's up to you to decide.

Master Yi himself is a very situational pick. I'm not talking about situations when enemy team has no CC or opposite, nah. CC is not much of a problem with proper positioning, timing of engages and Quicksilver Sash. The real threat for Yi is in bursts. Actually a half-decent Annie will spoil your day much more, than Amumu, Malzahar or somebody like that, and not just because of her stun, but the damage (which is hard to dodge even for Yi) she'll unleash on you in one single moment. But bursting champions themselves are not the biggest problem yet. As you'll be climbing in divisions, you'll notice that you're getting focused quicker and quicker, no matter how sneaky you are. Champions with boosts on takedowns (Yi, Katarina, Jinx etc.) are always the most dangerous, so they must be killed first, so that's why people keep saying such stupid things like "this noobish champ is useless in high elo". No. Not at all. I didn't manage to play in high elo much, but I know pretty much enough about this game to predict how will any situation escalate. And by the way, Riots are actually balancing the game for that "high elo", not for the silver-gold levels, so when one says that some champ is "too op" or "too weak", he just doesn't know how to play this champ or against him.
So why actually my build? Well, it provides more than enough damage and a pretty good sustain, a thing which "classic" Yi definitely lacks. You can say "but I'm assassin, I don't need to take damage at all". Well, if you will be that lucky to constantly get a team which can give you perfect initiations and override enemy vision with their own - then you're always welcome to build as much damage as possible. But keep in mind - a single mistake in late might cost you a game. So with a perfect team you will need to be perfect too, won't you? This build gives some sort of insurance. With nearly 3k HP and a huge shield you'll survive almost any occasion (not talking about those YOLO moments when you feel yourself totally invincible. 1v5 isn't for Yi, unless the enemy team is abusrdly dumb or unlucky). And that "classic Yi" can sometimes fail to even reach the enemy adc (you'll say it's just a bad Yi, but that's usually more of coincidence than someone's bad plays).
Anyway, again, the choice is yours after all. I just suggest a good looking build with pros like improved sustain and still a good damage output and with an only con that you won't be able to kill enemy adc in one second. In two much more likely.

Double Strike
That is your own AA damage modifier, and there's not much to say, except for advice to learn watching for it. In early good targeting the double strike proc helps to clean the jungle a bit faster and lose less HP. In jungling part I'll explain why it's so important. Overall the ability is damn simple, giving you and additional 50% dmg AA each fourth strike, while itself counting as a stack, so after the first proc you'll have a 50% AD burst each 3 hits. With your attack speed in late it will proc nearly once per second. Quite nice, isn't it?

Alpha Strike
First of all - that is not your damaging ability. Well, yes, it deals damage, and the damage is good, but if you used to play old AP Yi before rework or AD Yi right after rework (when he had 100% basic abilities cdr on takedown), you'll say now the damage isn't even close to those old days. Let me prove it by simple math - maximal AD available (including runes and without boots) with a decent crit chance (let's say 40%) is something between 500 and 600 attack damage. Well, maybe a bit more than 600. Its base damage is 165, and the scale is 100% AD. So let's say it will be 750. Maybe 800 if you'll go fullest AD possible (but you'll prolly have to sacrifise the crits part, so it isn't not effective either). I don't really know how the crit damage on Q is counted (Wiki has a weird formula which I didn't get), but if I remember right, when I tested it there was something about 450 additional damage. Let's round to 500. So 1300 pure physical damage. Even the adc's armor will block a huge part of it, not talking about the bruisers or tanks which are usually the 60% of the team. So let's say you get ~1000 damage per each hit and you have to time it right to hit 4 champions with it and then somehow survive, because 4000 damage is... well, it's just nothing for Yi. Actually even with full AD build you'll like the auto attacking much more profitable cuz of passive and E. Of course if you'll survive the first few seconds after the enemy team has spotted you.
So. I went too far, but my point is that your Q has to never be used a damaging part. Its primary goal is to dodge some nasty bursts and stuns or drop the turret aggro. The second use for it is getting into melee range to your target. Once I managed to travel the distance between outer and inner turret and get a double cuz enemy Kassadin flashed and ulted while I was jumping on him. I understand why he did it, but let's keep the mind games for later. So a dodge and a close-gap. What else it can do? Well, on early levels you might want to use it by CD cuz you don't have your CDR on takedown so you'll have to rely on 1 second CDR per AA. But even there I'd advise you to hold your horses. From other hand it's actually a damn good burst early, so if you are sure your enemy won't get away (e.g. Ezreal) or you won't need it to dodge CC (let's say Yasuo's/Janna's winds, but actually a lot of spells there). It's mostly intuitive feeling though, so you'd better try to improve that "alpha strike sense" by yourself.
And also don't forget that minions/monsters can help greatly in various situations. For example, you're ganking Fizz alone and he is using his trickster on you (because he actually can 1v1 you early with ease), and let's say you just meditated before engaging so you have your W on CD. The best option here is to Q the lane creeps so he'll lose a part of his burst. Same if somebody with ground-targeted abilities (like Xerath) ults or bursts with basic spells while you engage the enemy adc. He is sure you'll jump on him or on his adc so he'll try to shoot in that direction and he won't be able to react that fast if he doesn't expect you to "waste" your Q damage on creeps. Again the mind games, but wait with it a bit more.
Overall it's all just the basics you can learn yourself, and any player starting from gold (or somebody lower who plays Yi much enough) will tell the same, maybe better and shorter. But I'm here to learn you all I know (at least that part that I didn't forget) myself, so here it is (and also because it's damn rare to see a good Yi so some of those people will maybe learn something to improve their kit).

Meditate
Well, that will be even more obvious that Alpha Strike. The ability has two goals, none of them is primary. It heals and gives a great physical/magic damage reduction while channeling. Don't forget to use it in any situation. In early jungling take in on level 2 and use by CD. In lost fights, where you're not about to survive, you can win some time for your team, even if that's just a few seconds. And the sweetest part is to learn to use it in big lategame teamfights (well in any fights actually but it's hardest in late 5v5) to negate half of enemy bursts from champions like Lux. I gotta admit that I myself didn't master this part perfectly yet, but it mostly depends on your reaction and ability to think clearly in a big mess.

Wuju Style
Another damage modifier alongside with passive. Nothing much to say about it, except for advice to watch for it too. After a couple of Yi games you learn to E automatically when you go in, but remember, that the duration - 5 seconds - isn't that big, so you'd better not waste it before you get close enough to start hitting or before you use your Q if you are gonna to (especially in jungle, I used to EQ instead of Qing and then using E, which costs one-two additional empowered attacks). In late that usually doesn't matter though the true damage is very useful and you'll notice the difference between activated E and E on cooldown. But, as I said, it doesn't matter much, because you'll probably kill/assist someone in those 5 seconds and so you'll have another 5 seconds.
Ah, and one more thing - don't waste it on towers. They're not affected by true damage.

Highlander
So the ult, a skill which makes Yi such a beast. Nothing much to say about the passive, don't worry, in teamfights you'll hit each enemy and get a sure takedown if somebody will die. Now to active part. A lot of Yi players say (and I agree with that): use ult whenever you want. 70 seconds is not a big CD. Well, it is, but you won't notice it going off. Some even use it to more faster from base, but i wouldn't recommend that unless you have maximal CDR, which is not gonna happen in my build. Also keep in mind that duration is long enough but still it expires way too fast. So same advice as with E - don't be quick with its activation. Well, it's k to use it before Q (especially on Project skin to hear those amazing quotes), but not good to walk around enemies under ult waiting for a moment to engage - if you ult, you ult for some reason. From other hand, don't go full YOLO, it's always better to waste ult and sometimes flash to disengage than die. Yi without ult still hurts and dead Yi does nothing but gives the enemy time to bring their victory closer.
And a counter-advice to my previous advice. If you're in hurry to the dragon pit or a teamfight, then feel free to ult in 2-5 seconds reach from the target destination. It will let you arrive much faster and save the day (or die silly, but it can happen to anyone and anywhere).
Ah, by the way. Some people might not know, but under ult you are immune to attack speed slows aswell, so don't try to build Frozen Heart against Yi and don't be afraid of this item by yourselves.

So the build itself and why it is good. And why some usual things are not.

First of all, remember your main kit.
Skirmisher's Sabre - Devourer - well, because it works soo good on your champ that I don't even need to explain further. Red jungle item just because you don't need wards or slowing, and the damage reduction is a great thing, not talking about additional DoT.
Titanic Hydra - perfect interaction with phantom hit, AoE is always useful. Attack damage and Health are really good. The active interacts with phantom hit too, don't remember. That's the key item of overall build and you gotta make sure you biuld it first after devourer so you'll already have it when you're sated. I heard the AD part is gonna be nerfed a bit, but don't be afraid, it won't affect you hard (I may sound egocentric but it's so sweet they're nerfing my exact build a bit (cuz Yi's E is gonna be reworked in same direction); and yet I'm sure it's not enough).
Sterak's Gage - also a musthave item, but you may delay it a little to buy something like QSS or lifesteal. HP burst and base damage addition fit you perfectly (yet again, if his E active will be counter from bonus AD, that can be a nerf to the build, but again - a very slight one), but the main goal of purchasing it is its passive. It is just so damn good that I don't imagine Yi without it. I saw a few Yi players not going THydra but still buying Sterak. A weird decision but it emphasizes the value of this item.

Now you have to choose your boots. Overall you can buy any of them except for Sorcerer's Shoes. But i personally prefer Berserker's Greaves. Yea, that's not surprising, but actually I started to build it only after preseason boots changes. Before that Boots of Swiftness were my favourite, but in last time i changed my mind cuz of 30% attack speed buff and swift boots nerf. Also keep in mind that you gotta buy at least Boots as fast as possible, sometimes even before you end the devourer if you don't feel like it shall delay the devourer purchase.

So two more slots left, what can we have there? Well, actually a lot of things. I'll highlight the best choices in my opinion, but feel free to experiment.

Blade of the Ruined King - some say it's quite essential on Yi and in most cases I will agree with them. But it always was and still is an anti-tank decision (also anti-Annie and anti-Ryze), so there is no point to build it against a team of squishies. The GA would be a great replacement in those cases.

Quicksilver Sash - start with it after devourer if enemies have excessively annoying CC like Malzahar's ult or Rammus's taunt. Don't build it against single Amumu because you can always dodge his ult with Q or wait until it falls while using your W. Though the full item, Mercurial Scimitar is very good with its AD and lifesteal, exactly what you need. Magic resistance won't be bad aswell. Also the active is something you won't regret in like 95% games, be it dropping Lux's Q, Kalista's stacks or just a simple speed-up.

Wit's End - for cases when you need some magic resist above that the Mercurial Scimitar provides or you just don't wanna build it. This item has an on-hit interaction and a good attack speed boost. Also is a quite nice counter to Thornmail.

Youmuu's Ghostblade - with the changes on it, this item became a perfect choice on Yi. I build it rarely but when I do, it looks perfect.

Trinity Force - same as with Youmuu, changes made this item damn good for Yi. Don't be afraid to buy it if you don't know what to build and you feel like you want to improve all stats you can ever need. Totally worth the gold and the slot.

Guardian Angel - well, this item is always good to have, so yea, why not. If you feel too unsafe with enemy bursts, that's a perfect salvation.

Maw of Malmortius - another magic resist choice, but this one instead of attack speed and on-hit damage provides pure AD and much more survivability.

That list isn't over and i can also note any tanky item that gives you HP, and also highlight the Frozen Mallet, a decent choice by all the benefits it provides, though I build it quite rarely.
Now what's with not recommended items.

Death's Dance - this item is one of my favourite in the whole game and i started to buld it on Yi since its release. But then I understood that it's just not that worthy. It has no on-hit effects and healing part interacts only with physical damage, so devourer and wit's end damage won't be included. That's actually not much but still. You don't need the 10% CDR part and you don't need to heal on your Q just because it doesn't deal damage that much to make it visible. It's a damn good AD boost and all those benefits are decently usable, but I would rather say "Guinsoo, please" (see below my thoughts about it), than take the DD.

A lot of people like to go Statikk Shiv for I don't have any idea which reason. Attack speed boost isn't that good and you don't need the crits part much. As for me, I'd rather go Phantom Dancer if I would want to build crits with devourer for some reason. As a melee adc (yes, Yi is adc, and don't tell me it's same as marksman cuz it's not) he could really use some damage reduction instead of additional magic damage which he doesn't need.

Dead Man's Plate is another example of weird items people like to build. I don't see any point in that little burst on first hit, aswell as additional movement speed, because your own is just big enough. Health boost is great, and armor could really help against full AD team. So that's the only situation where it's valuable.

And the sweetest part which will make some of you rage - the Guinsoo's Rageblade itself. Let me make it clear before you close this guide - I don't personally like this item, but that's not the reason why I wouldn't recommend it. It's just not that good for him as other choices stated above (but still better than Statikk Shiv though). I mean, come on, ability power? 40 magic dmg AoE per hit? That weak attack speed stacking bonus? No, thanks. Nothing of it is worth the slot. And all together too. But again - if you like it, you are good to go. It's not bad at all, it's just not best choice without any interaction with other items. And magic damage on hit doesn't affect structures, so you will be bad at split-pushing (by the way, I am not gonna talk about split-pushing, but keep in mind that it's possible if team is smart enough to push another lane alongside with you and not to die too fast in 4v5; you yourself will probably be able to kill almost any foe, usually even two, with the exception of Jax (almost sure death) and Tryndamere (it's 50 on 50)).

Well, there are overall two major playstyles in early jungle - safe and risky. Safe way is simple - you start with hunter's machete and Refillable Potion. I usually start Gromp from both sides just because it's easier and I prefer to use second smite on red, but for safe play you may start whatever you want. Just take the buffs and start doing the small camps or watch for an early gank and get back as you get enough money for some utility items - advanced jungle item Skirmisher's Sabre, Boots, Hunter's Potion.

For the risky way I would recommend to always start Gromp by the reason I stated above. Take Meditate on level 2, heal, then AA blue golem and Q, then Q by CD but time it to dodge his AA. You'll survive with 50-150 HP, go to wolves camp and heal again, then AA, Q, do all the same. On level 3 take Alpha Strike again so you'll oneshot small raptors. On the way to their camp heal yourself by CD and then do the same thing, don't be greedy for Q and use it twice, so the big one is gonna die from your double strike proc. Then ward the most suspicious way to your red and get into the bush nearby. Heal and wait for Q, then do the same. If all is clear, smite it mid-fight so you'll be able to smite krugs as fast as possible. Go to them, heal, kill, take E. Then watch for the lane you're on (bot or top depending on side). If you think you can do something with it, you can try, otherwise go for scuttle crab and then to raports camp, they'll respawn at nearly 4:50 or 5:00, depending on how fast you were in that phase. Then wolves, gromp, another crab. After it you gotta come to someone's lane or try to steal enemy raptors. On lane even if you won't succeed, you can still take a few minions and damage the enemy laner so nor you, neither your ally will be unhappy. After that game flow can go in too many ways so I can just give a few useful advices.
First of all, don't waste your further time for gank with low chances to succeed. Your primary goal is to get devourer and level 6 ASAP, and by ASAP I mean the fastest possible way ever. After 6 you'll become a real threat, and with devourer (not to mention attack speed and on-hit damage burst) you will be able to keep helping your lanes while doing the main task - stacking. But also keep in mind, that if you don't gank at all, team's morale can get fatally low, they'll be blaming you, even ones who actually understands what are you doing. That will be even worse with some early gankers. So if you see enemy jungler arriving to lane, try to follow him unless you're on the other side of the map (and if you are, just gank the other lane to help to keep the pressure) or critically low in health/mana. After 6 it will become much easier though and you'll be able to deal with most of the laners (by deal i mean not to kill, but deal damage and survive; since you got your W, healing features from jungle item and probably the Hunter's Potion, loss of HP won't hurt you that much as it will to any laner without early sustain).

And also don't better go risky way against early counter-junglers like Shaco with ignite. You are veery vulnerable on first route, so keep an eye around and watch for enemy midder (he might come too and easily kill you) and ask your allies to ward the jungle entrances.

What's for Dragon, feel free to take it solo under ult at any time after 6 whenever you'll feel confident. AA him and Q a second later so you won't get kicked out, and then just keep hitting him. Don't Q him until the last strikes, he will probably kick you out (though not always, but that thing is really hard to time right). Rift Herald is usually not worth the time spent cuz of only 2 stacks, but with Tiamat or THydra you should be able to solo it aswell.

In teamfights you have to be damn sneaky unless you see a clear opportunity to pick up a kill and get out alive, which is always a good advantage for the next 30-60 seconds (even though you'll probably lose an ult and a part of HP).

Overall you'll want to be very choosy about the engagement time. You can't go in too early cuz you'll probably die for nothing. But if you'll be too careful, your team will die and you'll have to follow then and die too or defend your base alone vs 3-5 enemies. There can be a chance that you'll manage to clean up after your team, but that doesn't always work even if you look there and say "oh come on, they got no hard CC, i got my QSS up and that Garen is low hp, what can ever go wrong?", because something can always go wrong. First of all, you can't rely on your team in making enemies' hp low. Second - it's always faster to push in 3-4 men than alone. Even if those other guys are no damage support and a barely breathing tank.

Another thing I mentioned earlier - game of minds. It's not only about the teamfights, but about almost any game part, but I thought it'll be best to put it in here. An understanding of your enemy (Master yourself, master your enemy (c) Lee Sin; yes, I'll keep quoting different champs), the way how he plays, gives you a huge chance to succeed. The simplest part is to find out how nervous is enemy player or how is he afraid of you. My main battleground was gold for two years, and I've learned that even there a lot of people just instinctively press their escape spell when I use my alpha strike on them. They do it not on purpose and usually not only die but also give me an opportunity to follow the first kill, because they usually escape in the direction of other team. By the way, be very careful, experienced people will be doing this on purpose to bring you into the whole bunch of stuns and bursts. That's one more reason to keep your Q for a sudden moment, when the enemy doesn't expect that, so he can't react properly, because instincts will probably take over him. I hope it's clear with Q, but actually it doesn't end there. Use your high speed under ult to move spontaneously. Initiate the teamfight (in case if your team doesn't have a good engager or he's an idiot) and flash out of it to just come back in a few moments later. All has just one purpose - to make the biggest mess ever. Not the most skilled team wins, but the one which does less mistake. And the less they understand what's going on, he more they don't know what to do and the more they fail. Skill itself just determines how fast and good will you use enemies' mistake for your own profit and how long will you keep a clear understanding what you have to do.

Well, and sorta close thing to what I just was talking about. A great way to screw the enemy's mind up is to surprise him. All that positioning and vision stuff is done for that to be achieved, but I'm not here to teach you how to ward. Here i talk about the bursts you can provide. You see, one more good thing in my build is that you can burst people really good. And even if you lose the damage after that (not much though), they lose much more faith in themselves, because their confidence is practically equal to their HP percentage. Yea, I might be not fully right, but overall it is so. The opposite side of it that if you are sure you can kill an enemy but not sure if you'll take him if he'll start to flee, you'd better keep the bursting part for dessert. Just... just keep fooling people. That sounds nice and tastes even nicer. Once I got a good position in teamfight and managed to catch enemy Udyr so I killed him in 1v1, after that I had to chase others and kill them one by own from our inner turret on mid till theirs (while their outer turret was up). So when I killed the 4th one for quadra, enemy Ez appeared out of nowhere and Ed into me being sure he'll kill me (he had his right, I was under tower focus and on less than 100 hp after his engage). But happily i managed to dodge his Q and drop turret aggro for a second with the Q, and activated my hydra which luckily went off cd, and then.. then I just AAd him and killed him for my glorious penta (shh, don't say Yi pentas are easy. I know, but it's still warming the heart). For my own surprise, i managed to take nearly 800 HP with just one AA (I had only hydra and sated devourer and almost 200 AD). Good thing the shadowplay was on so I recorded and rewatched that. It happened that i had just everything up for that hit - phantom hit, double strike, E active and so was the hydra. For 2 items and 200 AD that's a great result. Even though it was more luck than anything else, I've learned that managing to put all the available resources together into one single attack and yet not losing anything is another key to success. What's surprising in it, you'll ask? Well, imagine your adc wiping out in two strikes from half-tanky Yi or your Tryndamere dying before he ults just because he didn't expect the 50% HP burst (yea, happened not ever once). Pretty scary, huh?

Also you'll maybe catch yourself (not even once in a single match) on a thought to trade yourself against two of them, but after hundreds of such heroic engagements from me I can surely say - it's not worth it. I'm not even talking about the risk to die in vain not killing anyone. Don't die for the double if you can get a penta (or at least an ace). In those cases when you really want to kill someone, just remember Yi's joke about key to immortality. It may sound egoistic, but Yi is clean-uper, he's best fit for serving this role and you'd better keep doing it, even if some stupid adc will keep whining about "Yi, why you don't give us kills and wait in bush to ks". You can't fix the minds of those people so just keep using your own to bring yourself a victory.

Just to underline what I said in beginning, it's all very situational, so the best advice I can give is similar to Kha'Zix's lifestyle - adapt. I can't describe even 1% of all possible situations you'll have to face, so your tactical mind and your gaming experience is your best friend there.

Anyway thanks for reading that far and I hope I was any useful in helping you with Yi. Remember, the sweetest part of the game isn't victory, but the process, especially those triumphal moments with comebacks and absurd ownages. You'll be told that "Yi is worst champ", "so noobish Q right click omg" and tons of such statements. I agree that his mechanics are easier than Riven's or Yasuo's, but after a few serious games against enemies who actually know what they're doing, you'll understand, that Yi isn't easier than any other champ, yet the infinite possibilities lie before him. Yep, that was another his quote, now from Project skin. This time the last one. Thanks for reading, live long and prosper.

P.S. And remember, that THE LEGEND NEVER DIES. Dark Souls fans will get my point though many would think Garen fits that meme better (and he actually does, but this time Yi is our hero).