The uncut/animal cruelty-free Cannibal Holocaust (with the turtle and the perfectly sync'ed music), Salo 120, the Auguest Undergrouds (pretty snuffy), Man Behind the Sun, 8mm, Necromantik (romance with the dead, cat scene).
I Spit on Your Grave and The Last House on the Left are 'shocking' because they are full of women being beaten and raped. LOL @ the wet yourself scene in Last House.
Aftermath isn't shocking, it's about some dude that looks like Mr. Bean who has sex with dead bodies, then bring some part of them home for his dog, Crazy Spain. The Guinea Pig movies aren't that good IMO.
Also it's NOT a film but Snuff: A Documentary About Killing on Camera is very interesting, some guy explains about some really hardcore stuff he saw, it's shocking itself and real.