Senator Palpatine/The Emperor - Because he was the only
character that was greatly acted in both the PT and OT. Ian McDiarmid freaking owned that part in the PT. His performance is one of the few saving graces of the PT. Plus, I still get shivers during the Palpatine scenes in RotJ. Great character. Great acting. Simply the best.
And just for funsies... My least favorite character would have to be Boba Fett (Yep, he's even lower than Jar Jar). He's really just a whiney emo kid whose "guy from which I was cloned" was killed so he went off and moped. Then, later, he shows up to capture Han, but was only able to do so after Vader completely incapacitated him in carbonate. And, as if all that weren't enough, he ends up being killed by a stumbling blind man. Talk about your whiney, pretentious, premadonna, Paris Hiltons of a character. I mean, even Jar Jar killed a couple of droids, but what did Boba Fett ever do? Absolutely nothing! Completely. Worthless.