2) Jonaleth Irenicus (Baldur's Gate II)
3) Cavil (Battlestar Galactica)
4) Senator/Emperor Palpatine
5) The Joker (I thought Nicholson and Ledger both did a great job with the character in different ways).
Plenty of other good ones in the above choices, but those are the ones that stand out as most memorable to me.