If I had to absolutely pick only one? Scout. Between the incredibly irritating sound clips for him and the fact that I can't play scout because my computer can't handle that many fps, I hate him.
Most of the other classes I don't really have too much of a problem with. Spam is rage-worthy but that's the same with every class. Demos, soldiers, lucksmans, spies spamming dead ringer.
Natascha heavies aren't too bad because the only map where heavies actually feel overpowered right now is on Mesa. And you switch to lucksman and pick off the heavy and his medic every time he tries to spawn camp you. If they fight unfairly, fight back with broken. Dead ringer also works for that.