Here is my list in no particular order:
- Session 9
- Eden Lake
- Antichrist (The Criterion Collection)
- The Strangers
- Martyrs
- Irreversible
- Cache (Hidden)
- A Tale of Two Sisters
- Frontier(s): Unrated Director's Cut
- Infection
I should also mention that the most disturbing film I have ever seen was not a horror film at all but a documentary called Jesus Camp. That one bothered me like nothing else, and I still haven't worked up the nerve to watch it again.