The Master (Doctor Who TV: The End of Time) (since he is changing his heart after perusing the doctor for centuries it would be higher if it (probably) wouldn't only have been temporary to get revenge
Jean-Luc Piccard/Locutos of Borg (granted he didn't have any real choice in the matter neither to nor from being a borg but the "treachery" of the character is breathtaking)
Anakin Skywalker/Darth Vader (On the list because it is a classic, but not higher because you can see both changes to and from evil coming a mile away)