Palpatine
Obiwan (I dislike him in AOTC for some reason)
Han Solo
Grievous
Tarkin
Quigon
(expanded universe)
Mara Jade
Thrawn
Jorus C`Baoth
Pellaeon
Exar Kun
Yeah, hard to pick just one. The Emperor in RotS was extremely well portrayed imo.
Hope Zahn will write more about Kinman Doriana too.