FFI: I guess watching my monk kill Chaos in 3 turns was pretty fun. Warmech was fun too but had too little life to be any challenge.

FFT: There were a lot of fun fights but anything including a major character (Dycedarg, Wiegraf, Vormav, Zalbag, Elmdor etc...) was very enjoyable as were the many tactical battles.

FFVII: The weapons were cool but using KoTR it lost the fun- some day I'll play them without it. I like the cinematic feel of the final battle and the fights to that point. The turks are always enjoyable opponents.

FFVIII: Seifer, and the set of bosses in the final dungeon rock.

FFIX: Kuja and Ozma, I've also enjoyed when not pissed off at most fights so far in the level 1 ozma challenge.

FFX: I think the Seymour fights are excellently made. It's been fun helping various not rpg oriented friends through the various times he fights you.

FFX-2: Well, yeah not much here. I liked beating Paine up in the den of woe- it needed to happen sometime.

My favorite's probably Trance Kuja as he was such an awesome opponent and I've always been so bad at FFIX that he could pose a challenge.