I really like both The Exorcist and Rosemary's Baby, though more as dramas than as horror films. The Omen is pretty good, too.
But I really disliked The Exorcism of Emily Rose.
Not because it was religious propaganda--The Exorcist is conservative-religious propaganda to its core, but it's a brilliant film.
What I disliked about The Exorcism of Emily Rose is that, first, it just wasn't a very good movie, and second, I thought it was dishonest propaganda.
It pretended to be "fair and balanced," but it wasn't.
The Exorcist, by contrast, makes no hypocritical pretence of giving the liberal-secular view a "fair hearing." Its intent was to scare people back to church.
I don't care for that message--but then, I don't care for the message of Triumph of the Will, either. And I have a copy of that movie in my DVD collection as well.
Actually, it was very important for author William Peter Blatty that the person who directed the film version of The Exorcist
be objective and agnostic on the subject, not necessarily a believer, the better to ensure that the results would be more credible. (BTW, director William Friedkin happens to be Jewish and not a Catholic.) I think that the objectivity that Friedkin brings to the film contributes to the film's disturbing nature.
I honestly didn't get the feeling from The Exorcist
that anybody was really trying to "scare people back to church" (although I can certainly imagine that as a side-effect!) In Friedkin's own words, The Exorcist
is a story about inexplicable events. And God (or who/whatever) only knows that life is full of them...