Harley Quinn (Batman: Arkham Asylum)
While Harley is blessed with generous womanly proportions, she is only good for an attractive silhouette. Because any shred of sexiness she possesses is ultimately destroyed by her penchant for heavy horror clown make-up. If Marilyn Manson had a Ronald McDonald kink, he'd look a lot like the picture on the left.
Add a large dose of sadism plus a nails-on-chalkboard voice and her sex appeal dwindles to nought.