Salo:120 Days Of Sodom is the most disturbing thing I've ever seen, and I've seen it all including the fully uncut version of A Serbian Film, Mordum, In A Glass Cage etc... Gore-wise, it is easy to sit through, but it hit a nerve with me psychologically. I was depressed for probably two weeks after watching it.
It is set during WW2 in Italy. Basically four nobles openly abuse innocent children in a mansion for 120 days. The cruelty involved is sickening. Many rapes occur both off-screen and on. Victims are ridiculed and killed if they don't co-operate, or punished further. They are told they cannot defecate until a certain day of the week, where the combined waste is presented as dinner which they are forced to eat. They have to act like dogs and thrown meat, some of this has nails in it which results in some internal bleeding - all at the nobles pleasure (the smile of one of them is sickening). Towards the end of the film you expect the children to get revenge - instead they are simply rounded up, blinded, scalded and murdered in the most painful ways possible. One kid has his eyes blinded, another has her forehead cut open and pulled back exposing her skull. The nobles also had a sick fascination of burning the chidrens sex organs with fire while they were at it. I had to take a shower after watching this movie. Highly recommended for anyone who wants their mind taken off things for a while.
Another movie which shocked me is actually Hostel 2. It has aged quite well. Some of this stuff is disturbing in how sadistic it is. The scene with the woman with the scythe is horrendous as it insinuates she is pleasuring herself as she cuts her victims up.