KEFKA (Final Fantasy 6) - He does so many horrible things, he usurps the powers of gods and uses their power to rule the world, smites an entire town because one person said he was a jerk, he's just a very hateable person.
THE LICH KING (World of Warcraft: Wrath of the Lich King) - I hate him because he has a great plan. His plan is when he realizes adventurers are running around Northrend, he LETS you work your way through the content, power up, defeat his most powerful minions, just so he can raise YOU, the player, into undeath and have you act as his general in order to conquer Azeroth. Yeah. It's a great Xanatos Gambit of a plan, and it ALMOST worked.
HERR MAJOR (Hellsing) - His plan is simply to spread war. And by god he makes it prime directive to be a complete monster of a person to accomplish that goal.