I'd probably say Seymour. I recently replayed FFX and enjoyed the game more than ever before, yet Seymour was one of the aspects of the game that really didn't go up in my estimation. There's little to no surprise from the player when he's revealed to be villainous, he's not particularly interesting and about the only enjoyable thing about him is the boss fights against him and that lovely battle theme of his for the fourth battle.

Sephiroth isn't terribly amazing either, though Jenova itself is pretty freaky. Ultimecia is really quite weak and always, in my view, seemed to have just been tacked on at the end. A lot of the criticisms of Seymour probably just as equally apply to her, but at least she doesn't have as much screentime as Seymour. Toss up between the two of them, but I must say Ultimecia.