Oh man, Tod Browning's "Freaks" wins the contest every time, for me anyway. Here's a blurb from THIS WEBSITE:

"Among classic horror films, FREAKS stands alone as one of the most grotesque and the most controversial. Rarely seen since its 1932 release, and banned in Britain for thirty years, it has achieved cult status as the masterpiece of the macabre.

The film features a cast of actual sideshow freaks, human beings of every conceivable physical aberration. Yet it soon reveals that the normal members of the traveling carnival are the true monsters; pitiless, conniving and murderous. When one beautiful trapeze artist discovers one of the freaks has a small fortune, she lures him into marriage. But when she and her strongman lover plot to kill him after the wedding, the enraged freaks defend their friend and take revenge..." [plot-spoiler warning after this line -bs]