Whats the best build for Yasuo?

Yasuo's passive (doubles his crit chance) is very important and helpful. If you want those items, get them after Statikk Shiv, Infinity Edge, and Boots. That would end up being a very glass cannon-like build, but possibly successful.

Mercury Treads/Berserker Greaves/Ninja Tabi Statikk Shiv Blade of the Ruined King/Bloodthirster Infinity Edge Dead Man's Plate Banshee's Veil

You want early Critical Strike Chance for your passive.

Go static shiv for item and rush boots as early as you can but the trinity force second items if you doing average and buy the infinity item second if your snowballing HARD (like 7-2) get the item you didnt get as your 3rd item the last 2 items are usally very split i prefer going the blood thirster as oppose to the botrk mainly because it adds to yasuos assasin component but there both good items to buy 5th DONT GET BOTH THE ITEMS ABOVE BECAUSE YOU NEED ROOM FOR ONE MORE DEFENSIVE ITEM usally i go deadmans plate but randuans or banshees works as well