BackFire
Blood. It's nature's lube
Chances are you won't be scared in a trivial way during your viewing of a film, like me. However, there are plenty of films that will mentaly haunt and disturb you, most likely.
Henry: Portrait of a Serial Killer - Eerily realistic and forboding, great great movie. Even if you aren't disturbed by it, it's worth watching.
Cannibal Holocaust - Cheesy, but powerful. Great music that makes the graphic scenes even more stunning and shocking. Topped off by real animal deaths that will disgust everyone.
Irreverisble - The end all, be all of disturbing films. It has a 10 minute long rape scene, done in a way that captures the true horror of a rape (or I can imagine). Disgusting scene that brought tears to my eyes. Also has a very graphic death scene, involving a Fire Extinguisher and a guys head. Watch it, it uses the formula of Momento but not just as a novelty as it was used in Memento. Had a purpose here. Anyways, probably the most disturbing movie I've ever seen, besides maybe....
Salo: 120 Days of Sodom - This is the quinessential video nasty. You will have great trouble finding this flick, but when you do you'll know where the hype and disgust comes from. It's about as close as you'll get to watching a real snuff film. It involves kids eating shit, forcing gay rape upon them, and eventual murder. Horrible, but in a good way.
Some others....
I Spit On Your Grave - This is another movie with a horrid rape scene, but it's a bit sillier and over done in Irreversible. But there is a horrid castration scene that induced my gag reflexes.
Last House on the Left - This won't disturb many people these days, watch this before the rest of the ones mentioned. If you watch it afterwards you won't be affected probably. Despite the fact that it's one of the most innovative and horrific horror films during it's time.
All of these films should get to you in some way or another.