Exploitation/mondo films are all I seem to watch these days (so I was sort of surprised I didn't like Grindhouse more than I did), but I'm more into the Faces of Death variety. I liked Cannibal Holocaust pretty well but repeated viewings made me realize how heavy-handed the moral of the story was.
No one's mentioned the Guinea Pig series yet. The second one is the only one I've seen and seems to be the most popular, partly because Charlie Sheen reported it to the FBI because he thought it was a real snuff film and also because it was a big inspiration to that kid who kidnapped and mutilated little girls in Japan some years ago.

