The Exorcist. As with Rosemary's Baby I remembered this as more camp than serious. But, again, I was surprised by how good the movie actually was.
The gradual possession of the young girl is done very well, and by the time she is fully controlled by the devil it is utterly horrifying. What really gave the movie nuance for me was the character of the priest, Karras. He was so sad and dark, and his struggle added a complex emotional element without which the movie might really have just been camp.