Cannibal Hollocaust:
http://en.wikipedia.org/wiki/Cannibal_HolocaustFound it on a list of the best horror movies, so decided with a name like that, it couldn't be bad. After watching it with some friends, they wouldn't talk to me for a while. Its so good it was banned in a bunch of countries, and the director was charged with making a snuff film because some of the death scenes were beyond the scope of 1980s special effects.
But really, if you are into slasher movies like SAW, this movie goes way beyond that, I wouldn't reccomend it for the faint of heart, or those with a weak constitution. I wouldn't say it is the
best movie I've ever seen, but it was the most impressionable.
More in depth on the director's snuff accusation:
He was found not guilty, but not before being arrested and serving a bit of prison time. What happened was, they wanted to get media attention, so what they did was after the movie was shot, he told some of the actors to go into hiding, so when they weren't seen or heard from again, people started speculating he had actually killed them. After being arrested and forced to prove that he could recreate the scenes in question, he had the actors come out of hiding, and was let off, but was still convicted for obscenity charges.