ZPowers said:
A couple no one has mentioned:
Visitor Q, which to me is probably more disturbing than Audition or Ichi the Killer.
Dead Ringers. In certain scenes, probably particularly for women.
The original Last House on the Left is fairly intense.
And though I dunno if they'd crack my top five, a lot of folks find Requiem for a Dream, most of David Lynch's filmography and Clockwork Orange somewhat intense.
Others I've heard of but not seen:
Salo, or 120 Days of Sodom,
Aye, thanks for reminding me. Good list of some of the best of the bad stuff. Miike is a hero of mine, so I'd like to add one more to that list:
Gozu (2003)
Really. You won't believe your eyes.
If we go that far back in time, I feel the need to add the original I spit on your Grave/Day of the Woman (1978). That one extends the horror of Last House on the Left in the random, almost casual brutality of "normal" young men, not escaped rapists and killers. It's invasive and brutal and will stay with you for a while. But, like Last House, it also allows you to think about what happened. To me, I like the empowerment that eventually, brutally follows the being victimized. Then again, they're both but movies, and reality is so much more harsh and brutal, so that in real life, the drama usually follows the plot up to where things get unbearably bad, but there's no revenge, no catharsis, no getting even. So, because they are movies, they allow for the sheep to be transformed into fiery beasts indeed, and that transformation is, in a way, just as horrible as all the blood and gore on screen. Traumatizing experiences can indeed make people grow, but they can just as well just bend or break them.
As for Salo, we have it on Laserdisc. It's pretty much a must see once you've decided to make up your mind about the world and anything in it. Pasolini forever. He also did some remarkable bits that are meanwhile little known (maybe because they're not horrible enough), such as Theorem/Teorema, which is basically the original Visitor Q, or The Gospel According to Matthew; good stuff if you can stand a guy talking at you for prolonged periods of time.