+1
There's a short film from '97 that disturbed me like no other: Cutting Moments.
8MM was kinda disturbing when I first watched it but, having since scene what could be called the film's predecessor, I found Hardcore to be even worse.
I'd be surprised if you hadn't already at least heard of it, but they say Hostel is pretty bad. I disagree.
I haven't seen Saló, but that film is always paired (at least in my mind) with the French film Irreversible. Another French film: Baise-Moi. I haven't seen any of these.
I watched Cannibal Holocaust about a year ago. Sure, it was kind of disgusting, but nowhere near my first recommendation.
Men Behind the Sun is a Hong Kong film based on a true story (the details of which have been censored in American books). I've long been familiar with the story, but have yet to see either this film or any of it's three sequels.
August Underground, August Underground's Mordum, and August Underground's Penance are nasty. I started watching them a few times, but never got through it. Not so much disturbing as distasteful.
Nekromantik was sickening, but hilarious at the same time. The necrophilia scenes were unlike anything I've ever seen in a movie before.
It depends on whether or not rape disturbs you (doesn't really disturb me, so I can watch films which feature rape no sweat), but ones which off-hand I'd say would be the most disturbing are those in The Last House on the Left, Ms. 45,and I Spit on Your Grave. There's also a rape in Cannibal Holocaust.
Needless to say, horror is my specialty.