Take into consideration he got buffed 4 patches in a row and half of his old counters either got gutted or nerfed hard. I don't even think diana counters him anymore just because of the amount of buffs he's gotten across the board. I'm guessing kata doesn't either ( i used to love how easy that lane was) with how hard she got hit with the passive nerfs and her overall rework. Also would help if you can think of a champ that can kill him as quickly as he can kill you (considering he's the only assassin aside from kha and nocturne that can basically kill you before you can react if you don't have a zhonyas up every 30 seconds (late game) and that you can't buy qss to counter his ultimate either. Talon maybe? I know the lane is hell pre 6. Yasuo is also a skill lane and you also have to remember the qss nerf which helpe yas out. He can still windwall most of his combo if plays his cards right, but thats a long walk.
Seriiously though, who the hell counters him riot has been dead set on making him meta for how many seasons now??