Pieces a.k.a. La Noche Tiene Mil Gritos is the slashiest, trashiest, most womderful bit of blood and gore I've seen in the genre

OP: Considering you've seen all the ones I would have said, I guess I'll have to go with the people who said "Rosemary's Killer" aka. "The Prowler". It was actually a brilliant movie and an extremely gruesome one at that.