Any and all hypermovement champs. It ruins gameplay, and removes skill on the other player (to a degree, less the higher rank). Its introduction also intensified the toxicity in the game, because it attracted more young players and flamers with the playstyle. It also caters perfectly into the snowball effect the game has, that is so pronounced that ppl rather forfeit than complete a game.
The sad thing is, they are so highly effective, because the meta is instakill. It might get more "outplays" in highend league, so its visually better to watch their games... but in the actual games, they pretty much ruin it instead. Of all i knew playing the game, only those under 20 still play. Everyone else moved to tft, aram, ttl when it was there. And slowly anything mature went absolutely away from the game. And my friendslist is pretty vast. Snowball and toxicity always being the reasons why.