I would say that Palpatine is the most powerful of the Dark Lords in the movies. Which other villains do you know that suceeded in almost entirely destroying the heroes, corrupting the Chosen One, taking over almost the entire galaxy, and almost suceeding in becoming the supreme ruler of the universe, all while slowly corrupting the Republic from the inside out?

I don't read the books or comics.

For the video games, Revan wins. <spoiler>The main character of KOTOR is actually Revan himself. He's one of the few Sith that fought not out of greed or lust for power, but to try to save the galaxy. He's also one of the few Sith that have managed to turn back to the light side of the Force after falling to the dark side. Destroying the entire Sith Empire and the Star Forge as nearly a one-man army is also pretty damn impressive.</spoiler>