My vote for greatest horror masterpiece would go to the Exorcist. Other horror flicks I would put in the masterpiece category would be, Psycho, The Shining, Night of the living dead and Halloween, there are others but these are the few that I think stand out. The Exorcist isn't my favourite horror flick, far from it, but I think it has some important factors that make it stand out from the rest. Besides being the greatest horror masterpiece of all time I think it's the scariest horror film of all time, I don't get scared by films but even I can see this movies creep factor. When I was younger this was the first movie that would enter my mind when I thought about horror, I personally think Night of the living dead is the most influential horror film of all time but when you combine all the elements , Acting, cinematography, dialogue , special effects, story telling, mystique and scare factor I think The Exorcist takes first place in terms of masterpieces.