For my money, Grievous wins! Upon first viewing, I really dug that guy. What really intrigued me was the coughing. Without that, it would have been just another one-dimensional standard issue bad guy for our heroes to kill. But that cough is what sold him as a mysterious multi-dimensional character. Just like the Boba Fett of old, I want to know more about his past, but so enjoy the intriguing mysterious personae, I wouldn't want to ruin it by learning too much. We all saw how AOTC destroyed all the coolness
from our mysterious ESB Boba Fett and I'd hate to do that to Grievous.