I powerfarm the figure 8 method. I get 4 health pots to speed up my feral flare stacks. I stay in my jungle for two clears if I can. I gank whenever there is an opportunity to to get fed, only if its worth it - meaning our team is guaranteed 1-2 kills. I try to look for a coup de grace opportunity with Yi. It usually happens at a dragon fight and you can kill 2-3 guys sometimes and get dragon. I rush Phantom Dancer after Feral Flare and Berserker Greaves. Not the biggest fan of Tabi or Merc Treads since you delay your power spikes with those, and you need to carry the team on your back after ignoring them for a decent amount of time. I like Bloodthirster since running out of the jungle with a huge shield is powerful and it synergizes with feral flare, meditate, and a defensive item or 2 if you decide to go bruiser. 20% Lifesteal is extremely helpful instead of 10%.
Master Yi hits his attack speed Cap really easily. He only needs 105 attack speed at full build due to Frenzy, Fury, and 29% from runes due to getting a free 80% from Highlander. Yi scales extremely well from the crit/movement speed from Phantom Dancer. It gives you the most attack speed out of any one item after Feral and Boots making your post Feral Flare game really strong. You give up Ruined King's lifesteal, but you get more damage with a full stacked Feral Flare. The benefit to rushing Crit also helps you if you get extremely fed. Occasionally I would skip Bloodthirster and go right into Infinity Edge. Extra AD is nice for more true damage.
http://www.lolpro.com/season-4/1506-mastery-calculator#3qTZ38vM2HQX
I like 21-9-0 on Yi since he goes hard and it synergizes with BT. Warlord is nice for burst, baron solo, and it isn't as win-more as the weavings. I like Warlord for bursting someone down quick and getting the reset. Good for pushing inhibitors faster. Warlord scales your true damage too. I use Legendary Armor because sometimes I think making a defensive item or two is neccessary to survive vs tankier teams. It compounds with scaling armor and magic resist runes. I don't like Block since he is in the jungle too long and that is the same reason why I like Bladed Armor. Wouldn't use Attack Speed Glyphs since I feel like they are a waste here.
I like Teleport but Ghost, Cleanse, and Flash are viable too. I think Ghost is better than Flash, but Flash is broken and it is extra good with Ghostblade and Ruined King. Vs CC you can use Cleanse with Yi and just cheese through it. Pretty stupid in a lot of scenarios.
Bloodthirster is suprisingly nice if you have a Randuins Omen and Banshee's Veil. I feel like Yi gets so much free damage, vs squisher teams it is better to just three shot squishies with Phantom Dancer.
Berserker Greaves | Bloodthirster | Infinity Edge | Feral Flare | Phantom Dancer | Defensive Item or Last Whisper - Bloodthirster is basically your Merc Treads or Tabi with the extra lifesteal. Vs Squishy teams, I don't get Last Whisper. If my team is winning and we have enough frontline, I don't get a defensive item.
Berserker Greaves | Bloodthirster | Phantom Dancer | Feral Flare | 2 defensive items. - Bruiser Yi is deceptively tanky with Meditate, Feral, BT, and Dangerous Game resets. I generally go with the previous build though.
I don't use Spirit Stone because I don't like putting 700 gold into it. I like rushing strictly Lantern for the gold and so I don't delay damage. Yi isn't neccessarily weak after level 6 unless your team fed a bunch of kills. So staying in your jungle with a Spirit Stone all day is just not for me. I get Feral Flare at 14 minutes. Sometimes a little quicker or slower depending on how fed I get.
I don't like Ruined King + Ghost Blade since it is inefficient attack speed wise later on with Berserker Greaves and it delays power spikes. I find it better to rush actual damage on Yi.
Playing Yi that way is more than just viable though, so play to your preference and use whatever you like. Yi can split push or hard carry, so even if you go no defensive stuff on Yi you can kind of chess them out and out rotate them.
I feel like Ruined King is an item which is better when you are winning due to the Lifesteal nerf. I don't like how its damage diminishes against guys who build armor when their health is low and you just want them to die right away for the reset or to survive and get out because you know the enemy team is coming.
Look for Dragon opportunities constantly and responsibly. Dragon is a great contribution to your team while staying in your jungle. I max Meditate second since Yi is very frail and is easy to punish. Meditating baiting in mid game and guaranteeing a reset is really good.
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This is just how I like to play Yi. It is how I win consistently in normals with him vs silvers and golds. Diamonds and Plats are prone to getting stomped vs fed champions like Yi too. Doesn't mean this way is best, just wanted to share my thoughts and playstyle.
Ranking with Yi is harder. It is difficult to first pick him since I think he is a champion who needs to take advantage of a later pick. Otherwise he gets counterpicked and I think the reward is just higher if someone safer like Jarvan or Maokai was taken. Use in ranked at your own risk. People tend to rage a lot more while you powerfarm the jungle due to stress there.