Its definitely the best horror movie of all times. This classic was made way back in the 1970s and it still manages to scare the hell out of people even now. This one is the very definition of a horror movie. Directors should learn something from this classic.
This is how good horror movies should be made. As far as the plot is concerned, it was awesome for that time. One can still fondly remember a number of scenes from the movie (for example, the staircase scene and the climax were extremely good). However, if mistakes are to be pointed out, there is one. The first 30-40 minutes of the movie could be easily trimmed down.
Performances were terrific. The direction was innovative (at that time) and superb. No debates on "The Exorcist" which can easily be said to be one of the best movies of hollywood. Unfortunately, none of the sequels (Heretic, The beginning, etc) were half as engrossing as this movie.