Most Disturbing Films?

I'm a total movie buff, and I really enjoy hearing others' opinions on films. My question is: what, in your opinion, is the most PSYCHOLOGICALLY disturbing film you've ever seen? I mean, movies that messed up your head or made you think, not the ones that made you cringe or puke. They can be gory, but I'm interested in hearing your opinions about films that are disturbing regardless of blood and gore.