Easy --> Walter. ( aka The Man in Black, Randall Flag, The Walking Dude, among others...). He's just so... evil. I love him. He balances out Roland perfectly. I also like how he, like Roland, does not just exist on one level of the Tower-- he travels to different wheres and different whens. King uses him to link The Stand to the DT series, which is delicious, and I feel that Flagg is represented perfectly in both books.
Unlike some of the other villians, I've always thought that he had more of a "human" element... like, somehow, I thought he would redeem himself someday. And in a way, I really don't think he was wholly evil... he's not a flat character.