Delores Umbridge, three times. She is a thousand times more evil than Lord Voldemort. He just wants to enslave or kill you...she wants you to admit she is right and that you like it. Kinda like Moochelle.
She was a fantastic villain. She embodied that hard-to-define evil that forces you to succumb to her will for your own good. That evil that, on the face of it, can't be recognized as evil by most people outside of her direct influence. In many ways, I found her to be scarier than Volemort since she's such a realistic portrayal of the kind of villains we deal with now.