The Exorcist it is a great movie with exceptional performances, even Linda Blair and especially Ellen Burstyn.
There are a handful of movies that immediately defined a genre upon release, and The Exorcist was one of them.
It's incredibly profane, however not at the expense of the story or to push buttons - it makes sense a demon would be so offensive. Just because you and others on these boards have become jaded with all the bad ripoffs like Hostel and The Human Centipede, movies that are simply meant to offend sensibilities, doesn't mean The Exorcist didn't do it right.
The story is rock solid. The mother is at a loss because she isn't a "believer" and knows her daughter is on the verge of being institutionalized, a place she would surely rot. So she has no reason but to turn to religion for help.
It creates interesting discussion: What's worse, the devil inside her daughter or the prodding torture at the hands of the doctors? And while the movie is very obscene, it can be argued that it promotes religion.
So while it may not be your number one horror flick, it shouldn't be a huge surprise that others find The Exorcist their top choice. Few movies elicit as many talking points as it does, even 40 years after having been released.
I have to say I'm pleased The Changeling made the list - a spooky ghost story for the ages.