Ultimecia, by far. When I made it to the fight with her, I really didn't feel like fighting her at all. She really hadn't done anything to make me want to fight her, unlike ExDeath, or Kefka, or Sephiroth, or Kuja. All she really did was "kompress time," and that didn't really seem to have much of an effect on the world aside from being annoying by closing up all the towns.