"The More Successful the Villain, The More Successful the Picture" - Hitchcock
What's the feeling on Hitch's quote? In this case, I don't think he meant successful in the sense that the villain triumphs but more that he or she truly inspires fear or hatred in the audience.
A good recent example would be "I Care A Lot." I couldn't stand Marla, but her character is what made it such a memorable movie.