Throughout the past few decades, this particular gem has been seen by many to be sort of the 'Godfather' of horror movies. In other words, the ultimate film for its respective genre. By today's standards, it could just be seen as another demonic possession movie, considering how many of those exist now. But I do believe you have this particular film to thank (or blame, depending on where you stand on the sub-genre) for it all.
This one goes back to 1973, and is based on a book that is based on a real-life case of supposed demon possession. It later turned out to be some sort of mental disorder, I believe, though many who have heard audio would argue until their blue in the face about it being real possession. What can you do? People think the Earth is flat nowadays, too. Either way though, if you can find the audio of the "exorcism", go for it, real or fake, it's pretty disturbing sounding stuff, and it ultimately made way for how bloody disturbing this movie is.
Your basic plot, if you don't know at this point, involves a lovely, innocent young girl name Regan (Linda Blair) being possessed by some demonic entity. It takes her over, abuses her from the inside out, makes her head turn around like she's an owl, makes her vomit pea soup, and gives her telekinetic abilities among other things. Her mother (Ellen Burstyn) calls for the aid of Father Karras and Father Merrin (Jason Miller and Max von Sydow, respectively) to exorcise the demon, and restore Regan back to her old self.
For my money, I would say that this is one of those horror movies that has aged extremely well over the years. In fact, back when I was still in high school, they released an extended cut of the film to theaters which seems to be the current takeover from the original. It's like the 'Star Wars: Special Edition' trilogy, but with no use of terribly aged CGI. This extended cut was quite literally just adding some scenes that were a bit too much for the time - the most famous of which has got to be her crab walk down the stairs while she spits up blood. Terrifying to hear, but iI'll be damned if I didn't rewind that and watch it again due to thinking it was badass (there might be something wrong with me).
Anyway, when all said and done, I also kind of consider this to be the film that made me face the horror genre head-on. Horror used to scare me growing up with the disturbing dialogue and imagery, and simply seeing clips from 'The Exorcist' would manage to put me on edge. So when I finally decided that enough was enough, my test was to watch this film, in its entirety, while home alone for a night. I managed, but that may have been because it was edited for TV. But with that said, nowadays, I find the genre to be a fun ride more than anything... except for torture porn, which I find completely cheap... also, yeah, I admit the whole demon/ghost thing is getting old as well. But this one is the one I'd probably recommend the highest out of any demonic possession movie, because hell, it's the original, and it's done completely with practical effects.