#3. Blair Witch Project - I saw this at a sneak-peak screening before all the hype began. It was still in that phase where people weren't sure if it was real or not. After that it got a bit too hyped-up and had the inevitable backlash against it. But it was still an incredibly powerful film.
I've always been a firm believer in that what's not seen is scary than what is. Once I actually see the killer, monster, whatever, the tension is cut in half. I think Blair Witch uses that perfectly. You only hear awful things and see weird stuff from the characters perspective, never actually seeing the witch. As dark fell in the film, I began to imagine horrible, horrible images to go along with the sounds, things that the filmmakers could never match. At the end, when the guy's standing in the corner and I remembered what had been said about why he was standing in the corner I got some of the deepest chills of my life, it took me hours to shake that feeling. I had trouble sleeping for a few nights after that, one of the handful of films that's done that to me since I was a kid. I still have trouble watching this film and have probably only seen it 3 times total. I think I don't want to ruin that feeling I got the first time around.