Must disagree with you here. I'll quote your own words: "the demonic horror genre only became a cultural phenomenon following the release of Friedkin’s movie." What you are saying is that "The Exorcist" was original; the demonic possession films since then are copies. This is reason enough to credit "The Exorcist" as the greatest horror film ever. Friedkin literally inspired a new genre, the "demonic horror" film. As to "The Shining," it was creatively intriguing but even Stephen King has disowned the film. Not a great legacy.