I love horror, most of my DVDs are horror but I can't remember watching a film that has really terrified me since I've been little. I guess one of the scariest moments that comes to mind is when you first see one of the crawlers behind the girl in Descent when the camera is on night vision, that was really freaky.
When I was little though, American Werewolf in London gave me nightmares

The part in the forest where he's lying on the hospital bed then just screams at the camera scared the **** out of me.