There are a few that are just too close to call for me.
Poltergeist, scared the crap out of me as a kid and now even more so due to reasons I wont go into now for fear of being dragged to the mental hospital.
The original Nightmare on Elm Street, brilliant movie that quickly became not only in my mind the best horror of all time but the best film of all time period.
The Entity, I am a man and this scared the crap out of me, watched it recently with the wife, she has only been watching real horror (things made longer than 10 years ago) since we got together. And it scared me more watching it with her, because of her crying.