I've seen many of the films mentioned and whilst the topic is obviously very subjective,, from my own personal perspective I've found certain films like Happiness, American Psycho and particularly early John Waters films like Pink Flamingos to be,, ok,, disturbing on some levels but very enjoyable due to the dark comedic and satirical elements. Irreversible is an excellent film and one I had no problems with.
However I did personally have to draw the line at two films - A Serbian Film and 120 days of Sodom (Salo). Believe me I did some research and even watched an initial few minutes of both films to gauge the styles of the two films,, but knowing the way my brains wired (intrusive thoughts / OCD etc) and the basic jist of those two I decided against watching them through.
I guess you could say that they don't count as I never got through them but I did the math enough to know they were too disturbing for me.
Then again I must point out that reading your average tabloid paper from cover to cover every day tends to be far more disturbing to me over time than 99% of what's considered "out-there" shocker/horror films. Needless to say I don't bother with them.
I'm not much of a fan of the normal horror fare these days anyway but was recently impressed with 'The Witch' ,, which was superbly shot, acted, and very tense and creepy without having to rely on the usual blood/gore/jump-scare techniques. Pink Flamingos and the follow up 'Female Trouble' are just downright hillarious gems of films imho. Whilst I get that they'd be disturbing to some,, the context and ways they're shot I presume would render them classics to the average Bluelighter,, not that I'd advise watching them with your moms that is!
I really didn't see what all the fuss was about regarding Human Centipede whatsoever, but was at least more impressed with the second one,, still,, a bit meh imho.
Anyway just some thoughts