I have to be predictable on this one. There's really no way around it. This is simply one of the best horror films ever made, and if Hollywood tries to remake it, I think we should all rise up and hang the executives en masse.

The Exorcist (1973)

This is the movie that really freaked people out. Seriously. It starts off so slowly, and builds tension with little things happening a little bit at a time, so you don't even realize the pace of the movie is accelerating throughout. The time between incidents shortens, and the scenes become more intense until you really aren't sure it can be handled...then BAM! A short, silent scene with the priests resting on the stairs before another crazy scene for the end.

After this movie sinks into you, no other movie will be quite as scary. This is the benchmark for all fright, IMHO. They make this angelic little girl into this frightening creature without making her into an unbelievable thing that would take you out of the reality Friedkin wanted to build. The voices, the acting, the creepiness...this movie has it all.