Mad Typist stole my thunder: I agree, that scene in "The Ring," when she crawls out of the TV, had me (and everyone else in the theater) screaming in terror. You saw it coming a mile away, but that almost made it worse (and made the stakes for the director higher) because you couldn't imagine it actually happening, or working. But it was crazy-making good.

(Don't let him in! From "Salem's Lot" 1979)

The first scene I remember really terrifying me was from the TV version of "Salem's Lot" (1979), when the child vampire levitates in the fog outside his brother's bedroom, beckoning him to let him in. Crushed, dazed with mourning, he lets him in -- and he ravenously sinks his teeth into his neck. I had nightmares for weeks, and I still prefer to sleep with something covering my neck. The murderous, monstrous child is always scary, because it's such a flip on our expectations and experience. But there was something, at least to a 10-year-old, so devastating about a brother being victimized by the very sibling he mourned and desperately longed to see again, that it tapped into an existential terror that nothing we believe in actually has any meaning. I saw that movie again a few years ago, replaying on Lifetime, and it still unsettled me.

Also: the great reveal in "Diabolique," which I saw in elementary school on PBS, almost gave me a heart attack. The terror!