Are League of Legends champions overpowered?

Naaah, they are not overpowered. LoL is basically balanced IMO, every champion got a counter. If you play DOTA2, you'll notice that the heroes there are way more powerfull and gamechanging than LoL's Champions. LoL is not a game that got overpowered character concepts, but in one patch or another some characters do get very strong because of buffing/nerfing.

But the problem of buffing/nerfing is not because of the game, every game that has some kind of balancing system has this problem.