Kiba and Masked Man, considering I found those to be the most challenging in the entire bunch.
Any mirror match, considering that means my version is better then my opponent.
Deidara, Storm 2 he was insane, now he's just a walk in the park for me unless I wasn't paying attention or the guy is just that good, yet some Storm 2 people still rely on him :).
Although I would have to be real that if somebody uses Masked Man/Kiba, next time after that I use Kiba xD. So the enjoyment goes away a little bit then because I then use an equal character to theirs.
But for me it's mirror matches because of above reason.