There is no one really OP right now. Of course there are some stronger champions and many useless champions, but there is no one gamebreakingly OP.
So who was your personal most hated OP champion in the past?
For me it is the old TF.
You walked through the jungle, trying to get to your team, then suddenly you see a sign above your head and you are unable to move till he stands next to you and onehits you? It is gone, hurra.
Ezreal is the only other one who got close in annoyingness.
I remember always loudly shouting "BAAAM" whenever his level2 q hit me for ~250 damage. And then you were chased out of xp range because of his 1200 range, or you died to his autoattacks.