I don't think you can write of looking cool that trivially. When you define 'best' villain what are you describing? Are you looking for most successful, most well developed? Then appearance is somewhat trivial.

However, I take 'best' to mean the villain that most enhanced your game playing experience. And to deny that appearance is a part of this is just plain ludicrous.

(Personally I think Kuja and Kefka were far better villain’s development wise; however neither of them I felt increased my enjoyment of the game as much as Sephiroth did.)