Presumably because of what it revealed about the U.S. national security apparatus?
Prior to seeing that film, for me it was “The War Zone”. Tim Roth’s film about incest. Incredibly good acting but the subject matter was way too hard to watch.
That is Sunday’s Game. While it is definitely disturbing, I still think it is an outstanding short film. It makes quite the impact in its short 8 minute run time and the acting is some of the most realistic that I have ever seen in any movie. I don’t think it is trying to be truly horrifying. It is even kind of funny in a really subversive way if you can get over the shock of the theme.
I almost took my now-wife to Mulholland Drive on our first date. A co-worker advised me against it :D.
In the uk, which claims to have been passed uncut, unusual a few years ago.
Well, my husband and I watched Looking for Mr. Goodbar yesterday. Neither of us found it all that disturbing, although I can understand how he might have been shocked way back in the olden days. 
It was actually pretty decent, though I’m not sure what the point of it all was. Maybe “if you treat yourself as trash, someone may dispose of you”.
I came into this thread to mention this film!
An important and influential movie for a couple of reasons. In parts surreal and in parts horribly realistic.
Truly great, and so truly disturbing, cinema. I don’t think torture porn can hold a candle to it.
Hostel is pretty messed up.
This one is technically a British public service announcement about train safety but don’t let that fool you. It has a demented plot, plenty of gore and is loaded with all the other elements you would expert from a horror movie.
This one isn’t quite as intense as The Finishing Line but it makes up for it by being even more bizarre in some ways. Here is another PSA about bicycle safety.
Another vote for Salo.
Also the rape scene in Irreversible. That was horrible to watch.