No one imo, you can't be the best with a champion that has a very simple kit with almost no mechanics. Anyone can master Yi within few games but in the end it's all up to your game knowledge like pathing, ganking, warding, strategys, planning, timing, laning, last hitting, reflexes, communication, map awareness, understanding the match up and so on. I sometimes play against champions like Fiora, i actually have no idea how her stun works, how long her spells cd are etc..

If you think there is a ''best Yi'' than you don't know anything about this game. I got my ass kicked many times against gold Yi players in mirror matchup, does that make them better than me?