Khellendros / Skie - Dragonlance Chronicles - The epitome of Lawful Evil in my opinion. Everything he does is for Kitiara, ever after she dies. And it's not love, just the ultimate in respect
Hexidecimal - Reboot - Megabyte has the cool voice. Hex has the insanity. I think the Cut-and-Paste episode was the best thing done in Reboot.
Room 1408 - Room 1408 - If you haven't seen it, yes a hotel room can be a villain.
Darth Vader - Star Wars - There were no prequels
Grand Admiral Thrawn - Star Wars Expanded Universe - And the great thing about him is that he never saw himself as one. He was just trying to bring order to the known galaxy because he know what threats existed in the unknown parts, and civilization can't stand up to them unless united.
Randall Flagg - The Stand - I think the only thing I don't like about him is that his personalty kept changing in each book he was in. But the one from The Stand was by far the best.
Achilles de Flandres - Bean series by Orson Scott Card - If ever there was a 14 year old who could take over the world by betraying every single country on the planet, this is the guy who would've done it.
Pazuzu - The Exorcist - I blame the acting for liking this one. Still the best possession ever portrayed.
Clown / Violator - Spawn - The most perfect suck-up piece of crap in existence, until you get up him upset
Lucifer - Constantine - He only had 5 minutes of screen time, but DAMN!