1. Voldemort (Harry Potter)
While Voldemort is pretty much known from all the books and movies about Harry Potter, he is actually a pretty straightforward villain/antagonist. Yes, there was some explaining about how he could have become so all-encompassingly evil, and he is in every sense of the word. He is as evil as they come. But, as far as antagonists go, he pretty much follows the cookie cutter model for evil villains. The only time we feel any remorse for Voldemort is during the flashbacks to his early life, during the times when he hasn't chosen the path of darkness and death. As soon as he does make his choice, all remorse is gone.
2. The Dark One (WoT)
The Dark One or Shai’tan, which is his true name, has a very dichotomous personification. Essentially, he is considered the most powerful evil being in the universe, but he is also paralleled to chaos personified and counter to the world’s creator (which plays a hands off role and is never brought into the series). If Voldemort is the stereotype evil villain, then the Dark One is his father. Not only is the Dark One an evil being, he is evil itself.
3. Odin (American Gods)
SPOILER - IF YOU HAVE NEVER READ AMERICAN GODS, SKIP THIS SECTION
Odin, in American Gods, is very different than the previous two antagonists listed here. Through part, if not most of the story, you see him as an almost Gandalf figure (the wise old man), except this wise old man isn't very nice and we put up with him. However, it is all a deception and he is trying to take over everything with the supposed enemy through means of trickery and distraction. He is a clever antagonist that I really enjoyed reading about.
4. Cthulhu creature and cult (14)
In Peter Clines, 14, the Cthulhu creature and the supporting cult are right up there with the Dark One. I wouldn't go so far as saying that the creature is the epitome of evil, but a very nasty piece of work. I really enjoyed the deception that was involved as far as the cult was concerned and knowing that they worshiped this nasty psychic gas bag made me hate them all the more.
5. Sauron (Lord of the Rings)
While Sauron is an extremely evil being, I can’t really put him up there with the Cthulhu creature or the Dark One. I only say this because neither the Dark One or the Cthulhu creature were anyone’s servant before they became the master. If you read THE SILMARILLION, you find out that in the beginning of Middle Earth, Sauron was a servant to an even more nasty dude. Yes, Sauron is a powerful villain, but when I learn that he used to be a servant to someone else, I imagine how bad things would have gone if that guy hadn't been banished.