The original, black & white version of The Night of the Living Dead
The original Texas Chainsaw Massacre
The Exorcist
More recently, I think the following all have their good, scary moments: Silence of the Lambs, remake of Cape Fear and Seven. Even "Cujo" had some pretty scary footage.
When I was about 7 or 8 years old, the local ABC affiliate had a late Saturday night program called "Wierd" which specialized in showing horror movies of the time. Very tame by modern standards but, to youngsters sitting on the sofa late at night with our cousins and with a blanket covering all but the top half of our faces, we sure got good and scared watching movies like Frankenstein, The Bride of Frankenstein, Planet of Blood and other such movies featuring the old dependable Christopher Lee, Peter Cushing, Bela Lugosi, Boris Karloff, etc.