Princess Azula
A few weeks back I mentioned that Azula was not only one of the finest villains in animated history, but that she was one of the finest villains ever scripted.
Part of the reason she succeeds so beautifully, of course, comes from the amazing voice work of Grey Delisle, whose delivery is always bone-chillingly perfect. According to some sources, at least one of her speeches was intended simply as a screen test, but was so well-done that the show creators worked it into an episode anyway.