The Exorcist also has that incredibly ominous and foreboding opening of Father Merrin stalking through the streets of Irag, going on a long and arduous journey, until finally coming face to face with a statue of the demon Pazuzu. I recently read the book for the very first time, and the film did a dynamite job of keeping that eerie, epic tone to it.
The Exorcist is my overall favourite ever, away from Argento. It's like a short movie on its own. The very opening where a boy is followed running through the scene of an archeological dig seems to have been done often since in that way, but I think The Exorcist was the first to do it.