The Shining, original and in the theater when my parents took us along as kids. Just wow.
I have to agree with others though, in my book The Thing is probably just as freaky as The Shining...ugh I don't know they are the only two that have freaked me out.
Oh, and the original Jaws in the theater back in the day - saw it while on vacation somewhere and I remember not going into deep water (that I couldn't see the bottom of anyway) for YEARS after that. I still would rather let a surfboard get away from me than chase it out too far if we get separated.