1. Martyrs- Very sick and hopeless movie, but very well made and the lead is incredible. Plus the ending is strangely beautiful.
2. Ichi the Killer-Sick masterpiece, and i love all of Takashi Miike's films <3
3. Irreversible- legendary for disturbing the living hell out of viewers.
4. I saw the devil- All sick korean cinema is special
5. Texas chainsaw massacre and Cannibal Halocaust- Just for being great and the original.
Of course there's Salo, A serbian film, and Men behind the sun- I dont like these, but they deserve to be mentioned just for being what they are
Most Helpful Guy
The British movie "Threads" is exceptional in this context, but no more disturbing film has ever been made. (Do not read about it extensively before you watch it. It's usually available for free online.)
It isn't a fantasy; it's a depiction of a reality which is inches away from our own. It will darken your world. It tends to profoundly destabilize viewers' perception of humanity.