The Exorcist

This 1973 film was based on the novel by the same name, which was written by William Peter Blatty in 1971. Blatty also wrote the screen play for the movie and acted as Executive Producer. The plot is based on the true story of the demonic possession and exorcism of Robbie Mannheim. Linda Blair plays the part of the possessed, and her portrayal is astonishing for such a young girl. If you haven't read the book by Blatty, I highly suggest it. The movie is also beautifully cinematic, especially the scene that depicts the arrival of Father Merrin, the exorcist. It is a Horror Classic in both print and film.

IMDb: http://www.imdb.com/title/tt0070047/?ref_=sr_1
Rotten Tomatoes Review: http://www.rottentomatoes.com/m/exorcist/