Know about/seen short clips in reviews? Human Centipede/Cannibal Holocaust. Never actually watched the movies nor do I intend to. Also the uncut version of Event Horizon I've only seen short existing clips for if it actually existed in full I'd watch it and it'd be it.
So that leaves Twin Peaks: Fire Walk With Me, which is basically David Lynch with absolutely no restraints. Full of fucky visual editing, incestuous rape, mindfucks, dreary and twisted music and imagery, it is just a horrifically dark drug trip that ends bittersweet as hell.
Also my favorite horror movie, love Twin Peaks