For me, it was Quan. I can't believe nobody else has had problems with this dude. He whooped my ka-boom-boom several times. Also, Gizamaluke was quite annoying in his own right. Silencing Vivi and Quina all the time. That's so rude of him! I eventually just started Chocobo-ing in order to get better equipment and beat him with that. But Quan was still my constant demise.
EDIT: Oh yeah, i forgot Tantarian.. he was a little beeeeeeach first time I fought him! It took me a while to figure him out. I hated that. Stupid little blue head son of a mother.....mumble mumble