Kayn
Honestly i was really excited about how Kayn's game changing adaptability would impact matches, but in reality it was a done deal from day 1. Rhaast is downright better than Shadow Assassin, straight up. Scalings are better, percent HP is op (psst Fiora i miss you) and if he hasn't ended the game with it then his huge lead will mean his items will outscale you anyways. The amount of damage Rhaast does for carries despite being the "anti tank dueling evil monster" is just undermined by the fact that it just turns into "evil monster".