Caligula- This movie has it all- as decadent as the man himself was said to be, starring ultraviolent clockwork orange's malcolm mcdowell as the sister fucking, citizen torturing, senator murdering crazypants emperor.
House of 1000 Corpses- Rob Zombie's gruesome, tongue-in-cheek, bizarre Grade-A B-Spoof Horror flick includes gratuitous titties, necro(soft)porn, mutilation of the living and dead, hicks, mutants, face wearing, gimmicks, gaffs, and torture galore.
Reservoir Dogs- See "The Ear Scene"...this flick made the name Tarantino synonymous with violence. my big cringe is when Mr. Blonde splashes that gasoline (apple juice) on the gap where the ear used to be... the best trick of the film is that, like in psycho, you never actually see the most violent part- the camera pans away to a sign saying "watch your head". Tim Roth got stuck to the floor by the puddle of blood he was drowned in. A re-make of a Korean film that Glen says is way better.
Suicide Club: Japanese schoolgirls, wives, and businessmen, take vengeance on co-dependence with mass suicide, because "the children" told them too after they tied all their skin together in a bag, while a glam rock serial killer takes advantage of the situation...and that's not even the most gruesome bit, we'll let that be a suprise (hint: it involves cookies)...
honorable mention: Kill Bill