So the series is filled with some seriously nasty villains but sometimes I feel they are only nasty because their powers tend to be really vague, look at the power of the Void, the crystals are it's antithesis but could a powerful sorcerer like Emperor Mateus or Kuja overpower ExDeath and take the Void for themselves? Could Sephiroth or Seymour's near immortality apply to world's like FFVI and XII which have more classical views of an afterlife? Hell could Nethicite neutralize Kefka's powers or ExDeath take control of the Cloud of Darkness?
Course this is all speculative nonsense, the real question here is which villain did you feel had the most vague powers?