Scariest (non-documentary) movie for me is a toss-up at any given time between Exorcist, Jaws, Texas Chainsaw Massacre, Rosemary's Baby, Alien, and Poltergeist.
One amazing thing about Poltergeist is that it has a body count of zero (humans that is... clowns, birds and imaginary steaks don't fare so well). A common thread between Jaws, Exorcist, Rosemary and Poltergeist is that all deal with imperiled kids and parents that are powerless to protect them. Hmmm...
I moved to the country right when Lost Boys came out and was scared shitless to go outside at night (I was ten, gimme a break). Though I don't think that one holds up as quite so scary. Except maybe Feldman's Macho-Man Randy Savage impression.