King Edward I from Braveheart. While historically a good king to English folks, he was ruthless towards the celtic welsh under Llewlyn ap Gruffyd and of course - the Scots. In the movie his portrayal is incredibly easy to despise - when he's dying in bed you just hate that he's not going to die violently.
Jack Nicholson's Joker from Batman.
Bif Tanen from Back to the Future.
Khan from Star Trek II.
General Zod from Superman - 'Come to me, Superman! I defy you! Come and kneel before Zod! Zod!'