Blair Witch. I always felt that if supernatural stuff was real, that it would be like it is in that movie. Vague, nebulous, and terrifying for those reasons. Not to mention I lived in the woods in an environment very like the movie took place in, which made it even creepier for me. Never forget being a little afraid to walk to my car in the morning while it was still dark out. Movie still creeps me out.
Cloverfield was the more entertaining film I think, but didn't have the emotional impact Blair Witch did. Shame they made sequels.
