Not only one of the greatest horror films, but one of the greatest films of all time. It may not be the scariest film of all time, but that doesn't mean it's not a masterpiece. It's such a well-directed film...so patient in its pacing and not blowing its load too early like many other horror films do, flawlessly acted, with brilliant editing and cinematography that really add to the eerie atmosphere of the film. Every shot, every cut is deliberate. The composition of each shot is perfect. Every element of each scene is utilized to its full capacity in each shot, and nothing in a shot lingers for too long, or too short of a time. It gives you just enough to make you feel uneasy.
The Exorcist isn't just a typical "girl is possessed" film. It's much more involved and deeper than that. For much of this film you question if Regan is just crazy or if she's actually possessed. Even during the exorcism scene, the priests aren't fully convinced she's even possessed. Which gets to the heart and core of the film: it's a film about faith.
Despite the religious nature of the film, there's quite a bit of science and psychology involved to make even the most die-hard religious people about how realistic their beliefs really are. And despite all this doubt and questioning of faith during the film, the exorcists put all their conceived notions, beliefs, and logic aside, take a leap of faith and try to save Regan in one of the greatest climaxes in all of cinema. It didn't feel like an exorcism just for the sake of an exorcism, it felt earned, perhaps even an epic feeling.