I think the fight with Xemnas was cooler, with the many stages, and the fighting with Riku, but i think Ansem (or Xehanorts Heartless) was a cooler guy. He looked like a mix between Sephiroth and Guildenstern from Vagrant Story (Both extremely cool bad guys), wheras that Zebra coat Xemnas wears in the last battle was a bit weird, in my opinion. The look is extremely important, i think.