As in the subject - what sort of character do you dread to play?
I mean character as in stats and/or abilities, not so much play style like cheese or fast/slow play. It could be the one that frustrates you, pushes you to the limit, or just challenges you the most.
For me, it's the topspin invasion. I haven't yet found a way to play against that effectively. They force you to chase down every point with the low bounce and precision angles, it really takes me off my game.