-Update-
In my opinion is Lord Sidious aka Palpatine aka The Emperor of Star Wars
He is vicious, cunning, insidious (that's the name's origin

), cold-hearted, puppet-master in all the stories and it's only objective is supreme power and imortality. When I first saw Vader at Episode IV, I thought that it could not exist a more evil and cold-hearted villian than that. Until I saw Episode VI and saw the Emperor. And that Lightening is cool. 8)
Darth Sidious in Star Wars Episode III Revenge of the Sith, played by Ian McDiarmid
Then I consider Lord Voldemort from Harry Potter novels.
He is evil, manipulator, cunning, very determined, doesn't looks at means to reach his ends, has a powerfull army of followers (Dark Wizards, Werewolfs, Corpses and Dementors) and he cut out his soul in 7 pieces so he could guard every piece in a different place, so when he died he could ressurect. He is also Racist and kills everyone in his way. He is the most powerfull wizard in harry potter world.
Lord Voldemort in the movie Harry Potter and the Goblet of Fire, played by Ralph Fiennes
I see much in common between these villians (they are also my favourite). They are both afraid of death and try to achive immortality.
Other villians I consider very evil:
Sauron
Krang