The Exorcist is one of my all time favorite horror movies. Why? First of all, William Peter Blatty's script is intelligent and well written. The characters and believable and complex. Many of today's horror movies pale in comparison, especially those of the torture porn variety with characters we don't care about who are only meat for the grinder. Instead of dumbing down, the Exorcist challenges the viewer to go on a journey that starts out slow and mysterious and then builds to a feverish pitch. Not much "happens" for a while so if you're a little ADHD you might not like this film.
Secondly, Linda Blair is absolutely amazing in the role of demon possessed Regan. Personally, I don't think the actress was ever the same again after this film. She channels something dark and evil here that no amount of CGI could ever replace. [Make sure you see the restored version which contains an additional 11 minutes of scenes, most of which are hers. I believe they were omitted because they were considered to be too shocking at the time.] The rest of the cast is stellar as well including Jason Miller in his iconic portrayal of Father Karas. Well known actors Ellen Burstyn and Max von Sydow also add lots of emotional depth to the film.
Lastly, William Friekin's direction of The Exorcist is wonderful. This movie could have easily been a very cheesy B-movie, but Friedkin keeps it creepy and disturbing. He shows us a few images that I don't think have ever been duplicated on screen. The scene with Regan and the crucifix still shocks me every time I see it. Friedkin was hired by Blatty to do this film after he saw Friedkin's work in The French Connection (1971). He was the perfect choice.
A brief word on sequels. Most critics agree that Exorcist II: The Heretic (1977) is a terrible film. I concur. However, Exorcist III (1990) is one of the most overlooked and underappreciated horror films of all time. It is just as good as the first and deserves to be seen by fans of horror everywhere. Blatty both wrote the script and directed Exorcist III. It's another well-made, intelligent horror film. Put it on your list.
The Exorcist can be seen in parts on YouTube. However, this is an insult to the original. Rent this baby ASAP and don't see it alone!
For more info check out the film's entry in IMDB.