Master Yi is a very versatile champion. He can specialize in extreme physical damage output, ability power, or like in my guide, be a Juggernaut . Perhaps there are many better builds, but this works very well for me, and honestly - it's a lot of fun. As Master Yi has a naturally high attack damage, with Alpha Strike and Wuju Style he can be a very fast jungler, a lethal ganker, and a Juggernaut . Using this build I have fooled my enemies many times to think I am squishy, and slain them.
Jungling is a lot of fun, obligatory in ranked games, and ganking is just awesome. This is a jungle guide, but you might also take a similar build route laning, just don't forget to get some sort of mana regeneration!
The builds aim is to make you able to effectively participate, and kill the squishies in teamfights. To achieve this, we boost Master Yi's resistance to crowd control, magic and physical damage, and also increase his life, sacrificing as little attack power as possible.
The defensive stats gained by this Master Yi build overwhelm the offensive ones lost. You will be much tougher to kill, and unless half the enemy team uses hard crowd control on you, and you haven't wasted all your mana, Highlander will make you escape. The build synergizes with Ability Power (AP) champions, as your high physical damage will force them to stack armor, and once your team commits to a fight, the enemy will have a hard time stopping you.
The Items chapter is right above the comments section!

Actual Stats at level 18, without buffs:

Pros:

Cons:

Offensive Tree:

Defensive Tree:

Viable Summoner Spells:

Greater Mark of Desolation and Greater Quintessence of Desolation together with your Masteries and Exhaust bring you very close to dealing true damage at low levels. It is very painful to itemize for armor penetration, and Wuju Style gives you good early game attack power.
Greater Seal of Armor is necessary for jungling, you need the flat armor early game.
Greater Glyph of Scaling Magic Resist gives a lot of end game magic resist. As a Juggernaut you want to have good magic resistance, otherwise you will be blown up by mages. Magic resistance items synergizes painfully with Master Yi, the only viable one would be Hexdrinker, so I consider these as must have too. Alternatively you might want to get some sort of mana boosting Glyphs, if you decide to lane its a must have.

Double Strike increases your auto-attack damage by 12.5% on average, but timing the extra hit yields surprising results!

Alpha Strike is a 4 target magic attack, scales with ability power. Its proc-effect reduces your need for Madred's Razors. For jungling its used to start clearing camps, in lane to farm and harass, and as a gap-closer in ganking. It strikes the four nearest enemies to the target in a medium sized area. If the target dies before the skill is executed you end up at your initial location. Use smite to kill a creep while casting if you only want to harass the enemy, but landing at the creep would bring you in danger. If executed well, you remain at your initial location. Otherwise if the target is alive when the skill is executed, you will be teleported next to it. Regardless, the target may die to the skills damage. Always take your low mana pool in consideration when casting this.
Meditate is a strong self-heal, scaling with ability power and resistance booster. The amount of healing it gives is too small at low skill levels to take it seriously. You might want to put 1 point on it when the incoming damage is very obvious, like Karthus' ultimate Requirem. The 1st point has to be put on it at level 13 as latest. In teamfights, when you find yourself focused heavily, use this to boost your resistances and get some healing, but only once the crow control abilities have been used. The channeling of this spell is interrupted by any hard CC, like silence, suppress, stun etc... If they ran out of CC and still focus you, the teamfight is as good as won.
Wuju Style gives you a lot of attack power passively when not on cooldown, which is doubled when activated. Maxing this gives you the attack power of a B. F. Sword and a half Pickaxe when activated. So why not max it at level 9? Activate it while you are casting alpha strike if you attack a champion.
Highlander is a very nice ultimate, and just perfect for ganking. Gives you insane attack and movement speed, you cannot be slowed and ruthlessly slice up the enemy. If you kill while its active, all of your cooldowns are refreshed. Do not reactivate it while its still active, its just a waste of mana. Alternatively if you find yourself in trouble use it as an escape tool. Use it on the Drake and Baron Nashor, as its cooldown is low, and the benefits of killing these creeps is huge. But make sure that you are aware of eventual ganks of you if you decide to activate it. Same with towers and inhibitors.

PRE-VOLIBEAR PATCH!

With the this build excellent early game defense you can begin jungling where you want, and the blue buff is not a must have on Master Yi. I will describe my main guidelines in jungling and early game ganking in detail.
Normally I start with the two golems. Stay in the bush facing the river until 1:30, this reduces the chance of early invasion. Ask your teammate kindly to do so too. Ask someone to pull the golems, and leave immediately. As soon as they are pulled, you alpha strike on them. If Alpha Strike didn't proc magic damage, you can attack any of them, and smite the one you focus once your teammate is out of experience range, the proceed killing the other one. If the magic damage did proc on one, auto-attack the other golem, and smite the magic injured one once your teammate is out of experience range. If magic damage procs on both of them, be happy. Now you have reached level 2, and mid will just start farming the minions. Ask your teammate to get the hardest form of CC available to them at lvl 1 (like Sion's stun), or some nice burst skill. If they have Ignite you have a very good chance of scoring first blood. Wait until the enemy gets very close to the maps mid (or even further), start running to intercept his way, Alpha Strike, active Wuju Style while casting, Exhaust once there. Its very important to get the Double Strike at the ghosts (just hit them a few times according to the number of strikes left to Double Strike), so your first attack will strike twice! After this proceed with Ghosts, then Wolves, go back for Boots. Next round if you see you have the possibility to gank, get the red buff and do so. If not, get the blue buff, and continue jungling. From now on you have to adapt (so far too!) to specific situations. A good Master Yi goes with the flow, and doesnt feed!

When you gank, do it like described above. Always consider: Is their Flash, Ghost or Exhaust on cooldown? How much slower are they? Do you have the slowing red buff? Do you have enough space to kill them, or they can flee to their tower? May their jungler or someone else manage to surpirse you? Would you survive a towerdive, and do they have a stun (and slow below lvl 6)? Is it risky for you, especially if you are on a killing spree? Also in case you are, and their support (or a random noob) happens to be alone, go ahead and towerdive! Either you get a kill, or the support gets the reward for icing you. Of course the best is if you don't die at all, but if the teams are close in terms of strength it's unlikely going to happen. If a fed Kassadin gets it, you are screwed!
When attacking towers, always activate Wuju Style and in case you assault on of the outer towers its almost always safe to activate Highlander. If you happen to complete the build in a game, you can solo towers without minions, losing 2.5k health points if you use both skills. But always be very careful when Highlander is on cooldown.

VOLIBEAR PATCH UPDATE!

The route described above is still very viable, but I feel that now it's better to start with the blue buff overall. With the weaker creeps, health replenishment buffs and your amazing defensive and offensive capabilities its a joke to keep the jungle cleared at all times. I start off with a Doran's Shield, and then depending on how the game evolves I either jungle until level 6 - your enemies will be baffled when they see you for the first time - or just as before, gank. In this case you need to keep your items up to date all the time, resulting in an overall slower leveling, and more unreliable gold income process.
The new patch rendered Madred's Razors even more useless for yi, as we now clear every camp with ease, even in one go at level 1.
Note: The red buff slow has been nerfed to 30% from 40%, but still pick it up - unless you want to waste your time against serious enemies.

Following this build, you have the power to participate very well in teamfights. To be a good Juggernaut (and play any champion well), you must continuously inspect everyones gear, know the summoner spells they have on them, and their abilities. Without these you will fail, utterly. Regardless how good our defense is, if you don't find the weak spot in their team, you will lose the match. Always look out for their maximum health (you can judge from the number of squares in the health bar without serious inspection), and as Master Yi you are interested most in their armor and damage potential.
Normally you do not want to initiate a teamfights yourself, especially until you don't have Warmog's Armor. Wait until most of the hard CCs have been used, and chaos begins to reign. This is when you step in and slice up the attack damage carry, or their ability power mage. Even if their tank is low health it can be lethal focusing on it, because their damage dealers will slaughter yours, then finish with your tank and support. It is very important not to begin a fight that is lost before it has begun, and if you ever find yourself turtled as a result of them dominating, don't engage until you all have build the specific items necessary to counter their setup.
To be a team worker you don't necessarily have to be a team fighter, Master Yi is infamous for backdooring. Juggernaut Yi can work very well with the team, but this doesn't mean you shouldn't backdoor in the right moment. Ask your team to push (but dont engage 4v5) top, while you go and backdoor the bottom. We can solo a tower and get away with 2k health points!
Always be positive and kind to your teammates; making them frustrated after a mistake can be the start of an avalanche leading to loss!

Doran's Shield: An excellent jungling item for its price, and you really need to spare gold, as jungling can make you end up with low income! Provides invaluable armor and health, and some minor health regeneration. Arguably you can get Cloth Armor and 5 Health Potion, but in this build I go back anyway to pick up Boots for serious ganking. Master Yi has more than enough attacking power to keep the jungle cleared at all times, so in my opinion building Madred's Razors provides only very little benefit compared to Zeal, for ganking. The attack damage is very little on it (the rest is kind of useless for ganks - similarly with Wriggle's Lantern), and the movement and attack speed on Zeal just ensures a considerably higher damage output, especially with maxing Wuju Style all the time, which is absolutely essential for successful ganking. So after getting [Boots of Speed]] go for Zeal, getting the Dagger first.
By now the Drake will be contested, and team fights will evolve, so its time to get Mercury's Treads
Your damage will be very decent now, so it's worth getting Vampiric Scepter, this will keep you on full health.
Build your Zeal into Phantom Dancer-which gives very much needed offensive stats; get Dagger first. Some people say Youmuu's Ghostblade is better, but as junlgers we need constant movement speed, and couldn't allow to use the ghost-blade's active just to catch up with the enemy, and Highlander gives us good boosts already. Althought the armor penetration and the attack damage are very nice on it too, we rely on criticals in this build, and Phantom Dancer just gives more.
Now we have crazy damage, despite having spent little gold! Now you can kill towers in matter of seconds - which you should do activating your maxed Wuju Style and Highlander!
Aim for Infinity Edge now! Once you complete this, your damage will be insane. Some say its too much gold, but it is really worth it, and while you get (the very strong) components of it, you don't lose a bit of competitiveness!
At this point Master Yi reaches a crossroad, and in this guide I took the way to tankyness! Why? Because your huge health pool turns to damage anyway with Atma's Impaler! Your resistances are high, so every point of hp is more valuable, and your damage will be overwhelming! I highly recommend getting Warmog's Armor first, then Atmas Impaler and finish off with Frozen Mallet. The slow, health, and damage synergize very well with your build, but it's not a must have as long as you can get the red buff! Feel free to replace it with very specific items, needed to counter the enemy team! More detail on these items is coming soon.Thanks for reading my guide! Credits for the separators go to JhoiJhoi. Feel free to point out mistakes, or suggest improvements! If you have a good score with this build, send me a link to the screenshot and I will include it! Please vote, so other will become aware of this build! Thanks!