How can you fight that which you cannot see?
Joking aside, the only hero I have an issue with right now is Omniknight, and I wouldn't even say he's OP really, just that it's ridiculous how much a good Omniknight can do. He can make your match hell, and in my opinion, he can even make up for carries who overextend too much, since he can just heal them, or even ult to save them and hope they're smart enough to back up.