A great villain has to be fleshed out just as much as the protagonist. If not, who would be frightened? Who would be compelled to turn the page to see if the bad guy gets the good guy. He is the dark to the light, the yin to the yang. The villain balances the story, he creates conflict. I chose Lord Voldemort because I love him almost as much as I love Harry. Without Tom Riddle there would have never been a famous orphan. Without him there never would have been 'the boy who lived' or a friend I could laugh and cry with late into the night when I read each of the books.
Any character who must not be named is intriguing. But my very favorite part is that Harry and Voldemort are intertwined, each is so much a part of the other that neither can live peacefully as long as the other lives. Voldemort is the dark cloud that hangs over Harry. Who or what is your dark cloud?
I can only hope I will create a villain that comes even remotely close to Lord Voldemort, with the layers and depth that he has.