Lots of good Horror Movies already posted. I can't think of a movie that actually scared me all that much, but not long ago I was watching TV late at night and a commercial came on for "The Exorcism Of Emily Rose". That damn preview freaked me out so bad, every time that thing came on after that I had to turn the channel. The part when she's walking down the street, and the normal looking couple walks towards her, and then their faces kinda melt and go all demonical on her.
Fuck that. I never want to see that again.
Even worse, we have a Movie Day every wednesday in our Happy Hour bar at work. Awhile ago the bartender said he was going to play that movie next. I asked him why:
Me: "Why the hell would you play that freaky thing?"
Bartender: "Oh, it's very good, unbelievably disturbing. I still wake up from nightmares in the middle of the night and have to smoke a cigarette just to calm down."
Me: "So you're gonna show it to a bunch of mentally unstable Happy Hour drunks?"
Bartender: "Well, I don't want to be the only person up alone at that hour of the night."
Nice guy.
Still, all time greatest "horror movie" would be, in my opinion: "Henry, Portrait Of A Serial Killer". There are at least two scenes that are very hard to watch, the whole thing just feels dirty and creepy. Very well made low budget flick.