Based on the terrifying novel by William Peter Blatty, this film won two Academy Awards including Best Adapted Screenplay. From my own personal experience of reading the book and seeing the film, I can tell you that both are frighting. Please note that if you are offended by profanity, violence and attacks against religion, you might have a hard time watching this film. I still recommend that you should watch this movie at least once.
Horror is one of those subjects that is very subjective, it really depends on the individual and what are their fears. What can be terrifying to one person, another may laugh at. Add to that most horror films were made as B films, meaning they didn't have good stories or acting, the studios would make them mainly to financially break even or give someone starting in the industry a place to cut their teeth.
This changed at the end of the code with Night of the Living Dead. Back in the 1920's there were a number of very good horror films, many from foreign nations which were not known for making good movies back then. By the mid 1930's horror was changed to monster movies or evil scientists as the villains, and would have a happy ending. By the 1960's, most horror films were just suspense movies that were not very good. There is the exception of Hammer movies from England, but those didn't get good until the late 1960's.
The reason why I bring up Night of the Living Dead is because it revolutionized the horror genre. If it wasn't for that movie, you would never get The Exorcist, or The Texas Chainsaw Massacre, or countless other horror films. Night of the Living Dead made horror films scary again. And it didn't have to have the happy ending.
This film is visually scary and psychologically frightening as well. Instead of jumping right in with the horror, it builds slowly. It progresses and gets worse and worse until finally it is terrifying. Great acting with Ellen Burstyn and Max Von Sydow and even Lee J. Cobb who I always forget that he's in this movie. Every time I watch it and see him I'm surprised and then remember that this was one of his last movies.
So while it may not necessarily end up on lists as one of the greatest movies ever made, it is one of the best horror films and getting nominated for Best Picture is an achievement in itself.