Darth Vader - Fell afoul of the lure of the dark side, this is more weakness than evil, so I doubt he is the most evil, and the fact that he redeems himself by killing the emperor does absolve him some what
Lord Voldemort - Seems to be evil for the sake of being evil, even in the flash backs to his youth, he is very evil. He just want's to watch the world burn, and Harry.
Sauron - Sauron is a 'token' bad guy. Tolkien made it so that Sauron had no redeeming qualities, so that he could be an undisputed evil villain with none of this "Well if his mother loved him enough, he could of been different" Stuff. So right from the initial writing, he was destined to be evil, he raises dark forces and kills thousands. Very evil, but not as evil as some of the others on this list in comparison.
Ganondorf - Ganodorf was king of a kingdom, so inevitably he wants what all men with power want- More power. So his evil seems to come from greed of more power it seems. I don't think that's as evil as Sauron or Voldemort, because again, it's like a weakness.
The Lich King - Arthas was a devout servant of the light, fallen to the Burning Legion's servants. It's ripped straight from Darth Vader, but Arthas never really redeems himself. And his acts are a lot more terrible than Darth Vader's. So The Lich King is more evil than Darth Vader.
Dracula - Dracula, if you are referring to the original sort of stuff, is written as a vampire, and vampire's are written as mankind's natural enemy. In the book, Dracula saves the solicitor from his wives, making him sound slightly heroic, but he's just saving him until all the legal procedures are taken care of. It's a good blend of man's enemy blending into man's society. Making them a social parasite, and I hate parasites, so that's evil yo.
Satan - Again, token bad guy. If you want to cast away all these modern culture references, and concentrate on what was told in the bible, you will know that Satan was the angel of music in heaven, then was cast out to the eternal plane of fire for questioning faith. Now, I don't know about you, I support free thought, so who is more evil? Satan for questioning faith, then being bitter about thinking freely? Or God for casting him to the plane of eternal fire for having a single unholy thought. I know what i'd pick.
I think Voldemort is the most evil. But is made a bit of a pussy by the universe he is in.