"August underground" series, "Bigfoot county", and "A Serbian film". These movies are definitely meant for people that 18 years old and older, although I watched them when I was in my teens. The disturbing part in Bigfoot county doesn't happen till the end of the movie.
I dont think in my book there is one 😬 I've seen a lot of horror and exploitation films and can't really claim one is scarier or more disturbing than the other. Cannibal Holocaust maybe one of them but I consider that tame lol. Are you lookin for a scary movie to watch or just curious about peoples answers? Human Centipede to me is a missed opportunity in my opinion. It is so terrible that no film company wanted it! IFC finally did and the man who came up with it? It was all a joke to him, I even said "did he write this because he lost a bet because this isn't a serious idea" I was right sadly.