Age is going to be directly related to your answer, because the best horror movies draw you in, let you feel like you are right there. And if a young fellow watched the Evil Dead original today, the impact would be far less than when it first came out. Same with something like the blair witch project. I first saw it at the drive in, without knowing a thing about it, and seeing how it was the first found footage movie, I was sort of thinking this was actual real footage. I mean people were actually gathering search parties to go look for those kids when that film came out.
So maybe pick your top three.
I will go with The Evil dead. This scene alone still sticks with me.
The Blair Witch, for the above reasons.
And I will go with the ring. It is still scarier IMO than any movie made in the last 20 years.