For me It would probably have to be Jaaaack Skellington. I never usually have other characters fight instead of Donald or Goofy (apart form Auron but he's in a different leage) but I like Jack and Sora's Limitbreak and it's Jack Skellington, how could you possibly not like him?

Oogie Boogie is by far my favourite Disney villian, as a whole not just from Kingdom Hearts. He's just so full of... pizazz. I always feel a little sorry when I beat him.