Hi, my name is Kasres on the North American Server. I've been playing Master Yi for a long while now, and am overjoyed at the rework. I figured I would share a little of my knowledge with you. I know you all will look me up, so I'll just say now that I am in Bronze 1 so you can ignore me if you will, but I personally don't believe I'm a bronze player. This build is a build that I got told to me from a friend, albeit in a rough shape, and I turned around and made it more useable. Now, with the reworks, Master Yi using this build (further on he will be called the Dunk Master) becomes a devastating duelist and Backdoor Champion. Also, this isn't my first guide, but I'm in no way experienced at this stuff, so please be patient.

Why Play Master Yi?
Master Yi is a finesse based, auto attack champion. He excels at backdooring during the mid game and is powerful in a team fight during the late game. His role is more of a Niche pick and he should not be played in every team comp. I can't stress enough how easy it is to shut down a Master Yi, and if you have a team comp that can't support him well (i.e. your team has no solid CC), Master Yi will crumble.

Pros and Cons
Master Yi has Several Pros as well as Cons.

Pros
[*] Strong Backdoor
[*] High Damage output in the late game
[*] High base move speed
[*] Amazing Pusher
[*] Can't be caught by most any champion

Cons
[*] Dies to hard CC and Blinds
[*] Has gotten a bad rap due to an extended period of uselessness.
[*] No CC
[*] Fragile

Master Yi isn't a Versatile Champion at all, despite the common idea. In my opinion, he is too fragile to reliably go top, and his early game is too weak to handle mid. This leaves his best role in the jungle. KEEP IN MIND : Master Yi does counter many people, and thus he can be very powerful in both mid and top, but only against those whom he counters. If you are in lane, I would highly advise you to use the Tanky Yi style, as opposed to the Dunk Master as Tanky Yi gives a stronger early game and lets you keep up with the wear and tear that enemy champions give.

Each of the builds I list here have a purpose, and thus should be played in a very certain time from each other.

The True Dunk Master is best in the jungle, and is rather weak out of the jungle early. Typically he's your go to guy for this role.

Tanky Yi is best in the top lane, where you need survivability. Still kinda weak early on, but not as much.

Burst Yi hasn't been tested, but he should be good mid lane or jungle, and he's there to take out squishies.

Time to talk about Runes. Runes are vital to jungling and are just plain useful.

The True Dunkmaster *NEEDS* armour yellows, as he is your jungler, and he can't take a hit without them.
I run scaling MR blues on all of my pages because they overtake the flats at level 7, and the jungle isn't scared of Magic Damage for a while early anyway. Another option is damage blues, however they are normally underpowered for attack damage. CDR blues are worthless as Master Yi's kit refreshes itself anyway.
Reds I run attack damage, as they have the most early effect in game, but Attack Speed and Armour Pen also work wonders.
I use MS Quints to empower my ganks and get in-between camps quickly. Attack Damage, Attack Speed, and Armour Pen Quints are all useful.

Tanky Yi will typically run with your standard tank page, so I won't go into too much depth.
I run Armour Reds because I find that most of the dangers we face early are physical, and so the flat armour really helps.
I also use armour yellows as they are, in my opinion, the best yellows.
Scaling MR Blues aren't bad, for the same reasons as in The True Dunkmaster.
However, I still use Move Speed quints to abuse Master Yi's high base move speed. Health is still an option, and damage isn't never a bad idea on Yi.

Bursty Yi uses a high damage page, yet I still use armour yellows to buffer his early game. Your choice of damage reds, of course.
The blues I still run scaling MR, but if you know you're going mid, you can swap them for flat if you think you need more power early game.
Where I use Moba's here, I don't need the Move Speed Quints. Once again, it's personal Preference.

For any form of Yi I play, I run exhaust. The reasoning behind that is Exhaust empowers your dueling, stops someone from fleeing, stops someone from chasing, or saves your life as you're meditating and you need only a second longer for backup to arrive. It's my go-to spell.

Flash is a god-tier spell, yes, but it's also a crutch. It allows you to escape or make a clutch play, but only once every 5 mins. If you over rely on it, it will be on Cooldown the moment you need it to save the game for your team.

Ghost is good on some champions, but not Yi. I mean, he kind of has Ghost already.

You need Smite to jungle, no question.

Ignite is another go-to as it stops life steal dead in it's tracks, as well as Fiddles or any other annoy champ like that.

Barrier is in a weird place, as it's useful in saving your life, but it's a one-trick pony. You can't secure a kill with barrier alone, it doesn't stop people from chasing, it's just a shield. That being said, it's a good shield, and can be taken in place of ignite if you need more defence or in place of any other summoner if you have reason to take it.

Most of the rest of the spells aren't good for Yi, or belong to support.

The True Dunkmaster is based off an infamous troll build, but where that build was silly and highly flawed, I've tried to make it viable. Due to it still being a Dunk Master variant, it still uses an absurd number of attack speed and crit items, but 4 Phantom Dancers isn't the wisest idea. The True Dunk Master is a build designed to duel and backdoor with ease, and has been honed for that purpose. I really don't think I have much else to add here other than about the boots. Your boots will be with you almost all game (unless you enter the Yi Zone, then you'll have sold them). However, you boot choice shouldn't be directly with this guide, use what boots you think you need for the situation.

Tanky Yi is a simple enough build to modify, as right now it's expensive and doesn't build any resists. Tanky Yi simply needs the Frozen Mallet (because who likes being Kited when your ult is down?), a life steal item, boots, and lots of tank. I build the two warmogs because I want Atma's to do a lot of work. That being said, one warmogs and a frozen mallet will still make Atma's give about 50 AD, so it's not that imperative to max out your health counter.
Tanky Yi is a very versatile build for Yi, and it can be reused for any bruiser. If you have to wade through a lot of AD, I'd suggest Hexdrinker/Maw of Malmortius or something like the new Spectre's Cowl for lanes like Nidalee or Karthus. High AD teams are normally countered by health, but thornmail isn't ever a bad choice.

Bursty Yi is something I haven't tried fully (normally my team feeds too hard for me to get a chance), but you should aim to build him like talon, zed, or any other assassin. Some feedback on what you all find effective is really appreciated. The only thing that I really suggest taking is the Sword of the Divine, as it will melt a carry in the blink of an eye with Highlander.

In the jungle, our early game is very focused on farming, as Master Yi has no CC to bring to bare to a gank. If your lanes have reliable CC, ganking is easy and should be done when you can to make your lanes win harder. Be that as it may, chances are, you won't have this luck or your lanes will already be doing good, so just farm the jungle. I don't suggest an invade as dying early will end your jungle.

In Lane, it depends on your matchup. If you are dominating, then dominate. If you're being dominated, hug the turret like it's your mother. I've found lane Yi to have mana issues early, so don't be afraid to buy mana potions as the lane continues. Also, never forget to leash.

By now, at least one turret will have fallen and laning phase will have ended. At this point, if you didn't get at least a kill or a few assists, you'll have a hard time keeping up, and you'll have to farm a lane to get your damage up, at least as True Dunk Master. This is where your back dooring capabilities come in handy. You can easily push a turret or two during a skirmish in another lane, and your move speed with Zeal and Quints makes it hard for an opponent to catch up to you.
One Caveat though: Don't back door unless your team can hold off 4v5s. If your laning phase wasn't too hot as a team and/or your team is comprised of squishies, then back dooring may not be good. Most of the time it's not an issue as you'll still have a tank and possibly an off tank, but just be careful.

If you didn't go True Dunk Master, then stay with your team. As Tanky Yi, you are the team's offtank, they need you. You aren't an initiator, but you do scare the pants off of the enemy carry and put as much damage on the most dangerous person they have.
If you are the Bursty Yi, wait until the team fight starts, then get to the enemy's main damage dealer (normally the carry) as fast as possible and burst that person with all your might. You're a one-trick pony, so don't be too afraid to back off after that if you are too low to keep fighting.

By now, your build should be fairly underway, unless you're losing hard. All forms of Yi should stick with their team, not be the engager if you can avoid it, and burst down the enemy damage dealers when the team fights take place. If there isn't a team fight threatening and you see a lonely squishy in lane, try to take them out, but only if you know they aren't baiting (namely, make sure you know where all 5 are before taking them out). Split Push if your team needs you to, as any form of Yi can easily take turrets by now.
Yi can carry a game if fed enough, never forget that.

In summary, Yi is a powerful champion in certain situations. Not much else you can say about that. The only thing to remember is that you can't just pick up Yi and be a god; there's no champion like that in all of League of Legends. You have to practice. You may feed a few games until you get the hang of him, but once you find your groove and which build suits you best, you'll be golden.

ANOTHER IMPORTANT NOTE: NONE OF THESE BUILDS HAVE BEEN THOROUGHLY TESTED IN RANKED, AND WHERE MY CURRENT PLACEMENT IS FULL OF PEOPLE WHO THINK BANNING THEIR COUNTERS IS VALID, IT WON'T BE FULLY TESTED UNTIL I GET OUT OF BRONZE AND SILVER. DON'T JUST JUMP DIRECTLY INTO A RANKED GAME WITHOUT TESTING THIS FOR YOURSELF!!!!!