Master Yi is the one champion that defines "carry". Played good can't turn siege of own base into killing spree near enemy nexus...not to mention that he can end battle with 30+ kills no problem ( my record with this build is 41 kills)

With variations of this build you can archieve ~400 damage, with slightly over 2.00 atk speed and 90% probability of critting with 270% damage.
This gives us:
2x(400*2,7)~ 2100 damage per second. Still not impressed? That is without ulti and active E.

Unless somebody wears really heavy armor you put them down in a second.

First of all, despite what your teammates may think: your not a god. Even with 20 more kills than anyone else. This build is extremaly squishy. You got no defense mechanism at all.
If someone survives your initial slashing and targets you- you run or you'r dead.

Prons

-extreme damage output
-you can kill enemy almost without slowing down your run
-enemy morale lowering (trust me-ppl run like hell when they see this green icon on map)

Cons

-squishy
-easily ganged
-sometimes hard to play in team fights
-item dependant
-no defence mechanism
-did i mention extreme squishy?

Runes
Greater Seal of Attack Speed - for attack speed boost early game.
All other runes are set for critical damage output, since your critical chance are going to raise to 90% this is another call for maximising damage output.

Spells
I take Exhaust to use both to defend myself against 2man gangs (exhaust one, kill other one, fatigue goes of kill another one), to run if someone cathes me low on health with cooldowns empty and of course for chasing if highlander is on cooldown at the moment.
Second one i take Flash or Teleport. Flash is usefull if you trust your team and you know you will have no need for cover someones lane in instant, you can focus on yout chasing/fleeing skills. Take teleport if you'r playing with unsure team and fell that you'll have to ofter relocate in instant to the other side of the map.
As for other spells: defensive and healing ones its the same situation as with masteries-why booster something that doesn't exist. Offensive ones takes more time to cast that it takes to kill enemy with your boosted auto. And the damage is irrelevant in comparison with your main damage.

Early and Mid game
For an early game I start with Boots and two Health Potion. This gives you mobility to get out of dangerous situations or to chase and secure this early kill. Potions allow you to stay in lane longer (since my build takes Meditate in the end. Farm creeps with Alpha Strike and try to aqquire some gold for Berserker's Greaves and Zeal. At this time you should be able to put some heroes in your lane down without much problems with combo of closing in with Alpha Strike then activating Wuju Style and then auto till you beat the **** out of them. At level six after getting Highlander and possibly Phantom Dancer you can litterally defeat anyone 1vs1. Combo remains the same: Alpha Strike, then Wuju Style and Highlander auto-bangbang, dead. Start accumulating kills on all lanes, when you have time don't push yet instead go jungle. Begin building your Infinity Edge. From 6 to 13-14 its just lane jumping, securing kills, maybe small pushing but not nessesarily.

End game
End game with this build is pure teamwork. Don't even try to go solo anywhere cause you will be gangeg in instant. Your role in teamfight at this stage is to stay in back/backdooring/flashing/bushing out of nowhere and secure as many kills as possible as long as you feel safe. When you feel that your HP is dropping to low - RUN. If your team has good CC and tank is doing good, going for a pentakill is more than possible. In end game even if you rush into enemy, team secure two kills and weaken third enemy as a kill for a team, but at price of your life- go for it. Because Good Hands give you shortened dead time and your team of 4 got 2 enemys against. And that means nice push for your team.

As end game items i go for:

and....last item varies.
Sometimes i go for Frozen Mallet if i think my squishyness is above, high already, treshold for this build. If you got problems with heavily armored tanks go for Youmuu's Ghostblade (which have build in weaker version of Highlander in it). If harrased by mages try Wit's End. If you have no particular problems go for Black Cleaver for damage output or another Bloodthirster for increased vampirism.

In fact i slightly modify this build every game I play and ecourage you to do the same. Thing to keep in mind are: speed cap is 2.5. If you reach it with items Highlander becomes literally useless, so i always try to cap my attack speed at 2.2 to leave room for highlander.
Same thing with critical chance...can't go above 100% right? ;)