Not a Review Necessarily: Is the Exorcist Still the Scariest Movie Ever?

Linda Blair as Regan MacNeil in The Exorcist (1973).

I believe it's a safe assumption that movie audiences, whether horror fans or not, know to some degree that director William Friedkin's 1973 movie The Exorcist was and still is an impactful movie. That's putting it in simple terms, but it's true.
The movie is about a young girl named Regan MacNeil (Linda Blair) who's possessed by the devil. Her very non-religious mom (played by Ellen Burstyn) has no idea what is happening to her daughter. With all avenues of medical help exhausted, she finally turns to a Catholic priest for help.
The whole demonic possession motif is so common in the horror genre these days. By "demonic possession," I mean the idea that someone can somehow be taken over by evil spirits, and the remedy is the intervention of a priest or religious person to exorcise that malevolent spirit. I'm not referring to supernatural horror movies about haunted houses or possessed dolls, or the like.
Before 1973, there was no movie The Exorcist could really be compared to. It was a new kind of horror subject for the big screen - a subject as old as Christianity, and older. To see a Catholic exorcism reenacted was brand new and terrifying. But audiences get used anything the more they see it.
A lot of inspiration springs from this one movie.
After the release of The Exorcist, the theme was repeated in movies such as Abby (1974), Beyond the Door (1974), The Entity (1982), Burnt Offerings (1976), Prince of Darkness (1987) and of course the sequels to The Exorcist to name a few. 
The cultural impact of the movie is intense.
Though horror movies centered on exorcism and the demonic have lingered on through the 1970s, 1980s, and the 1990s, the 2005 movie The Exorcism of Emily Rose brought the horror sub-genre back into a more prominent place not seen since audiences watched Linda Blair be taken over by the devil.
What these two movies have in common is what makes them stand out above the rest. They're both based on true events.
The Exorcist is based on the Catholic exorcism of a 13-year old boy back in the 1940s outside Washington D.C.
The Exorcism of Emily Rose is based on the Catholic exorcism of a German woman named Anneliese Michel back in the mid-1970s.
Both movies also have a documentary style about them, especially Emily Rose which focuses around a court case.
Friedkin comes from a documentary background. His previous film, The French Connection, has a similar feel to it. That's why author William Peter Blatty, whose book this movie is based on, picked him to film the movie.
That style adds to the frightening nature of each movie. It's an emphasis on the idea of "what if it really is true."
There's a ton of movies centering around possession and exorcisms released in the last 10 years alone - The Last Exorcism (2010), The Rite (2011), The Possession (2012), The Last Exorcism II (2013), Deliver Us From Evil (2014), The Exorcism of Molly Hartley (2015), The Possession Experiment (2016), The Crucifixion (2017), The Possession of Hannah Grace (2018), The Prodigy (2019) and more possession-based movies slated to come out in 2020. There's at least one for every year of the last decade! Demons are all the rage now.
To be honest, I find these kinds of movies repetitive and dull. They rely mostly on jump scares and grotesquaries to frighten audiences - contorted bodies, eyeballs rolled back into the head, scars and lesions, and a lot of vomit! Occasionally there's a good movie, but they don't hold a candle to both The Exorcist  and The Exorcism of Emily Rose. 
I first saw Friedkin's movie back in the mid-1990s when I was a teenager. I grew up in a Catholic household as my late father was a devout Roman Catholic. He attended the old rite of mass said in Latin every Sunday. He was never without a rosary in his pocket. And he owned quite a library of books on various religious topics including demonic possession.
I've read a few of these books on actual exorcism cases - Hostage to the Devil by Fr. Malachi Martin S.J., Begone Satan by Carl Vogl- an account of demonic possession involving a girl back in the 1920s in Erling, Iowa which lasted nearly a month - and An Exorcist Tells His Story by the Vatican's Chief Exorcist, Fr. Gabriele Amorth, to name a few. I found all these books to be very compelling. And of course there's accounts of possession and exorcism recorded in Scripture. So, being raised Catholic meant a belief that these things can and have happened.

And with that belief about exorcism, I was initially scared to watch The Exorcist. But that was me.
For others, it boils down to personal religious beliefs. Are demons real? Is there an eternal Hell where the souls of unrepentant sinners go? And can demons possess the living?
There's a lot to be said about the intense reactions audiences had when The Exorcist first came out. Some people vomited and passed out in theaters. Others ran out crying. Some non-Catholics ran straight to Catholic confessionals. What other horror/thriller movie had such an impact before 1973?
But audiences are used to seeing the sub-genre portrayed over and over again. For the most part, audiences have become desensitized to exorcism movies.
It's that documentary style in The Exorcist  that still holds up and helps set the movie apart from other such films. What also stands out is the focus within the movie of how the events surrounding Regan are effecting the two priests performing the exorcism makes the movie work. One of the priests is in the middle of a battle against doubt regarding his faith. The other priest is suffering from a heart condition. And now they both have to take on the devil himself.
It's not just a scary movie. There's a lot more to the story. There is so, so much for audiences to take away from The Exorcist.
Exorcism has effected real people beyond those who are supposedly possessed by demons. This movie addresses that, and that depiction is a scary thing in itself.
As Friedkin said in an interview, "It's not a movie about Dracula. It's not a film about the alien. It's a film about a real street in a real town, and upstairs on the third floor of this house is a real little girl who happens to possessed by a demon."
Meanwhile, other exorcism movies lean towards being monster movies. Despite my general dislike, there have been some good ones. Hereditary (2018) definitely pops into mind as a memorable and truly scary possession movie. But even that movie is more of a monster movie than The Exorcist. That's not necessarily a bad thing, though. Movies like that, and most other exorcism movies, are meant to scare the audience more than make them ponder the reality behind the phenomenon. The Exorcist and The Exorcism of Emily Rose have a much more educational undertone than most other sub-genre films.
But perceptions have certainly shifted over time. Compared to recent horror films not necessarily centered on exorcism but with scary visuals and such, maybe The Exorcist is a product of its day. However,  with the reality of the movie, along with knowledge that many religions (Catholicism particularly) deal with problems like this in real life, other possession horror movies just don't compare story-wise.
Maybe for non-religious individuals, the movie may lack in scariness. I've heard one non-religious person claim the scary aspect for them was how the mother in the film, Chris MacNeil, who isn't religious at all in the movie, and thinks her daughter is literally loosing her mind, finally has to appeal to a Catholic priest out of desperation because no medical expert could tell her what's wrong with her daughter. She places her trust in an institution (the Catholic Church) which she is completely unfamiliar with. Otherwise, for religious folks (Christians especially) it still holds true that it's the scariest movie ever. Demons and the possibility (though rare) of possession is a very real part of Abrahamic religions.