What are some really awesome horror movies?!?

Okay, so my friends and I are having a horror movie marathon this weekend for Halloween. I was just wondering what are some really good movies?! We're looking for like... top of the line. We love gore, like Saw. And horror like... Halloween. SOME PG-13 movies are alright, but normally they're pretty lame. SO, we want some ACTUALLY SCARY (because nothing really scares me) and PRETTY GORY movies. They don't have to have both. It could be one or the other, or it could have both. Just give some suggestions!