Look. I don't want to go into a crazy amount of detail as to why Master Yi is overpowered. He's insanely powerful if fed, his ultimate is cheap. He really is a game changer if played correctly. The main objective is to get fed and carry the team. The best way to do this is to have a friend to lane with and coordinate ganks early game and have them LET YOU FEED. Master Yi is not a difficult hero to play, but he can be difficult to master the art of stepping in and out of combat, switching targets, and running away when appropriate. Moral of what has been said so far, Master Yi is kind of a scared puppy, with a bite bigger than his bark if you are skilled and fed enough to take him that far. I hope you enjoy the rest of the guide, good luck and have fun carrying teams.

This build is based around a "Laneing Yi." You may be able to transfer the build to benefit a jungle build but try that on your own time. Now, Let's take a quick overview of how to set up a Master Yi pre-game.

Pros:
You are extremely powerful late game
Escape slows via Highlander
Great push hero for turrets
Easy to learn

Cons:
Extremely vulnerable to gank early game.
Squishy late game
Frowned upon for being "cheap"
Often focused by enemy team

Listen, Master Yi is an awesome hero. He's a monster of an assassin and an amazing pusher. He is probably one of the best carries this game has. But he is cheap, you will get hated on for using him. The only thing that sets a good Yi from a bad Yi is knowledge, practice, and skill. So show people you know what you're doing, get out there and make people hate you :]

Greater Mark of Critical Damage
Crit Damage
Greater Seal of Critical Damage
Crit Damage
Greater Glyph of Critical Damage
Crit Damage
Greater Quintessence of Desolation
Armor Pen

Why so much Critical Damage?
Infinity Edge Provides you with Critical Strike and Critical Damage increase, ontop of its +80 Attack Damage. The weapon screams for Master Yi and this build. Infinity Edge is merely adding more power to you swings with the high crit increase. With a high crit chance in late game, all of your runes, items, and masteries that provide you with bonus critical damage give you around 500 to 900 crits on enemy heroes (depending on their armor and such). You cut through people. End of explanation.

The last 6 mastery skills are pretty much where ever you see fit to put them. I use them in the utility tree for reasons I will go over further down in the guide.

You could also put skills into Resistance and Hardiness in the Defensive tree if you are looking for survivability.

Double Strike
This ability is Passive. Can be helpful if you start combat with an enemy hero while this is stacked to "6." this will provide a second initial strike at the beginning of the fight. Not that big of a difference but it has proven helpful.

Alpha Strike
A great way to close the gap on fleeing enemies or starting combat from bushes. This ability ensures you have the opening advantage.

Meditate
Heals Master Yi. Great way to recover health quickly and jump back into the fight.

Wuju Style
Great source of burst damage. Always use this ability first (unless Highlander is off cooldown, of course) if you are going in for the kill.

Highlander
Increases movement and attack speed. Resets all of Master Yi's cooldowns if he receives a hero killing blow on the opposite team. (Does not reset Summoner Spells like Flash or Exhaust) Probably one of if not the most over-powered ability in the game if utilized properly.

MAKING EXCEPTIONS
You can slightly tweak this sequence to include Meditate if you are fighting a lane with heavy harassment heroes. Simply check your level, then skip what you would normally level up and replace that one skill upgrade with Meditate. Then, continue the sequence again according to your level. An example would be at level 5, Instead of getting Alpha Strike get Meditate. Then at level 6 get your Highlander level 7 Wuju Style level 8 Alpha Strike and so on.

CORE ITEMS
You should have a finished build with these 6 items:
Berserker's Greaves
Wriggle's Lantern
Phantom Dancer
Infinity Edge
Last Whisper
Bloodthirster
Feel free to buy Bloodthirster before Last Whisper if you so choose. This is my preferential.

Why These Items?
Mainly, you have Infinity Edge because he is used as an Attack Damage champion. This Doesn't mean he can't be Ability Power, but I prefer high attack damage with high attack speed. You will see why once you try my build. Refer to the runes above for a moment. See all the Crit Damage runes and armor pen Quins? This is why we have all this crit chance. Popping your Wuju Style double your damage output for a short while. This is your burst, and this is why Highlander is so overpowered. You can literally burst over and over again. With the insane crit chance and crit damage runes you can 3 hit enemy champions when you pop your cooldowns. And if you can't 3 hit them, they wont last much longer, you have stacked attack speed as well. With this high of an attack speed, plus you Highlander attack speed active, PLUS all the Attack damage. No one stands a chance. Yi is a machine. And yes, he is very cheap. But who doesn't want to take advantage of this monster. Try him and this build out. You won't be disappointed.

Beginning Game
First things first kids, Master Yi is EXTREMELY easy to kill early game, as well as late game. Be prepared to get focused and pay attention to your surroundings and minimap for MIAs. Spawn with your team and pick up your Boots and ONE Health Potion. Head to your lane with your buddy or lane-partner. You can either hug the turret at the beginning or hide in the bushes and hope you can claim First Blood with your friend. Progress through the game taking creep kills for money and building your core items in the order listed above. (Remember, this guide is designed and built around being fed. If you aren't fed/dying a lot, you may have already lost your chance to use this guide to it's full potential.)
Early Game
Around the time you have bought your Zeal, you should be ready to start straying from your lane to begin ganking. Pay close attention to allys and learn their abilities as well, as they can be key to ensuring you a safe, easy killing blow. Remember that your Highlander ultimate refreshes all your cooldowns IF you land the killing blow. Continue feeding on stray heroes and don't feel shy to set up a gank on the other team. (this is why I recommend having a friend with a support hero be with you)
Mid Game
Once you has claimed your Infinity Edge you can pretty much count on landing the killing blows (as long as people are purposely TRYING to land the final hit for the kills). You Attack Speed is high enough from your Phantom Dancer and the critical change and damage will hack through players health base. The only problem you should run into now is if the enemy team starts to single you out to prevent you from any further feeding, and running out of mana from the constant re-activation of ALL your cooldowns after each kill. I recommend killing Blue Rune for the mana regen if you haven't already figured that out yourself. Continue to push towers by yourself with ease, and gank heros until you reach your Bloodthirster or Last Whisper (whichever you decided to purchase last.)
Late Game
Finally, you have completed your build. All of your core items are purchased, You are 3 hitting heroes, and people are trying to "1v1" you. You have truly made a name for yourself by now. At this point, no one should really stand a chance against you or your team anymore because you are probably carrying the team by a long shot. You can either walk into the enemy base, claim victory, and look for the next game. Or continue to farm people with ease and watch the dead bodies stack up. Happy Hunting!
Here is some pictures of my build's aftermath.
ThrothGamingDeninarki - First Time Master Yi[These Pictures are when I stacked 2 Infinity Edge . Through some guidance in the comments below, I have revised my build and replaced my second Infinity Edge with Last Whisper for the armor pen.)

Master Yi CANNOT initiate a 5v5 team fight. He will be focused if he is fed because he is squishy and will die very quickly. You are a threat and honestly you are probably their BIGGEST threat. So remember to coordinate with your team. Be nice, don't kill steal and if you do just apologize. Good teamwork will benefit the team and will offer you the distraction you need to sneak into the fight and kill their team before they can even realize you were there.

I hope you have enjoyed this guide. I've been playing Master Yi since level 1 and I love him. I also play Nocturne for jungle games but that's another guide. I hope you have learned something useful from this and good luck mastering this way of the Wuju!

-ThrothGaming

Thank you all for the feedback thus far! Please continue to critic the build so it can become one of the best on Moba!