Requiem for a Dream- I saw it when I was 16 or 17 late at night over a friends house and I thought it was very, very weird. I like weird movies, but it was a little too much for me. Maybe I was still too young, but the forced sex between the two girls (if I'm remembering correctly) really disturbed me at the time.
And speaking of disturbing, I saw The Descent this past weekend and it was the most gory movie I think I've seen that I can remember. And with how the first half of the movie is, you don't expect it. It had really on point sound effects that just made the gore that much more head-turningly gross. I liked the idea of an all girl cast not being sl*tty the whole time, but I felt like it was more of a disturbing experience than an all around good movie. But that said, I think it's worth seeing as horror films go.