I know you are going to be able to read this Warb, but for the record....
The Exorcist is not the scariest movie ever made I still say that crown goes to Shoah (because it is real) but it's a amazing piece of cinema and like the original The Texas Chainsaw Massacre it's reputation hides a very clever use of cinematic language to make you think you've seen more than you have actually seen.
Some of the sights are actually sounds.
Watch the film with the right set up and the subsonic tones and the little hidden noises will give you much more of a fright than any of the well publicised special effects, some of which are beginning to show their age.
But more than the scares the characterisations drawn out of the beautiful script from William Blatty by relentless, merciless direction from Billy Friedkin out a well chosen and driven cast are the centrepieces of the film.
There is only one sequence I would remove (people who have watched the film will know what power compels me to say that...it's a bit of a whoops moment in an otherwise flawless film).
I'm going to go out on a limb for a moment and recommend the so called Version You Have Never Seen (you haven't seen either version but it's the special cut).
Because it has a few scenes which I think are crucial to the story and I can't understand why they were removed, a moment on the stairs in particular (the talky one not the walkie one).
It's best seen in a cinema but with the lights down (and no trips to the bathroom) like it or not it's an experience.
I guess I don't need to warn you away from Exorcist 2 or either of the prequels but Exorcist III is a flawed (tampered with by the studio sadly) gem and in the same fictional universe is Blatty's masterpiece The Ninth Configuration (one of my favorite films ever).