No. 1 Heimerdinger
People tend to disagree on whether Heimerdinger is actually OP. Yet what everyone agrees on is that they do not enjoy seeing him in their game. He is one of the most frustrating champions to play against, precisely because of his ability to slip from you every time you think you are about to kill him. He can create up to three turrets and then just sit behind them as they farm or wreck enemy champions that venture too close. The ability to send Hextech Micro-Rockets allows Heimer to hit you with them from the safety of a mile away. If you do manage to surprise him and get close, Electron Storm Grenade can stun enemies which will help him escape; all while his turrets keep blasting you. Not fun, but very OP.
