How about ol' SATAN in The Exorcist. Or the shark in Jaws. Both of those movies were so above the standard for scary that years later when they were finally shown on commercial TV they had disclaimers about being unsuitable for small children.
I had to go back to the theater to see The Exorcist on my own because my girlfriend couldn't stay past the 360 neck turn. People actually got physically ill when they saw it the first time. When you think about some of the stuff in todays movies these movies seem tame.

Don't forget the little beastie in "Alien" too. People screamed out loud when that thing burst out the first time.