Of those who are referred to as The Dark Lord, Voldemort gets my pick.
Vader doesn't really qualify for the title, in my opinion, since he turned good at the end. I don't have enough information about Sauron, but he and Voldemort almost tied. It came down to the fact that Voldemort had nothing happen to him in order to turn him evil, he just went down that path. Perhaps Sauron didn't either, but I don't have that info. Lich King was a possibility, but he was simply taking revenge, so I didn't think he had the right motivations. Dracula didn't have much of a global plan, so he's out. Finally, Satan is pretty powerless according to Biblical canon, so he is really out.
Edit: I realized I had only listed reasons I didn't pick anyone else. Here's why I picked He-Who-Must-Not-Be-Named. Voldemort managed to use the darkest magic in order to remain immortal (something the other people on this list had handed to them), his flaw in maintaining this immortality was hubris, which is a classic Dark Lord flaw, and he was racist to the highest extent.