This will obviously stem back to what films you've seen as a child - as most horror movies viewed by adults for the first time aren't nearly as frightening as when viewed for the first time by a child under the age of 10. However, you have some very impressionable adults out there with the mentality of a child, so one never knows. Perhaps someone saw "Phantasms" and truly got the whim-wams.

So let's have it. For me, as I haven't been alive as long as some of our other visitors of this forum, it would have to be "The Omen" or "The Amityville Horror."