1. Oldboy- The moment when the main character realizes he committed incest disgusted me so much. I realize it's the point of the movie, the villain takes the ultimate revenge on the protagonist by destroying his life... the story is told so well, the film is made so well, but I was so disgusted that I don't think I can go through watching that film ever again... I tried once, but I stopped watching at a few parts
4. Antichrist/Nymphomanic (A Von Trier tie)
Years ago I would've put Schindler's List, Requiem for a Dream and Grave of the Fireflies, but I did rewatch them eventually. What I found weird about Oldboy is how many people recommened it to me with a smile on their face saying "Ohhhh! You've got to see it, it's so cool." A man sleeps with his own daughter. I remember feeling sick to my stomach at that moment.
EDIT: Some other films mentioned that I agree with- Elephant, United 93. I read about Salo, I don't think I'd be able to manage that. I read about Dogtooth which I know a lot of people here loved, but I also think that might be too much for me.