Regan's head spinning in The Exorcist. Keep in mind, this was in the early 1980's, as I was very young, and that was still some pretty horrifying imagery at that time.
The Elephant Man scared the hell out of me when I was in preschool. Obviously, I didn't get that it was the tragic story of a very unfortunate man at that young age, I just walked in and saw his image on the TV and was seriously traumatized for a few years.
The scene in which Murphy is murdered in Robocop disturbed me a great deal as a kid too. It's not so much that the scene is filled with horror and suspense, but it was the first time I'd really been shown a hero or good guy being blown to pieces with that kind of realism. It was just a darker side of reality I had never really considered before up to that point in my life.