To me this is still the single greatest horror movie ever made, because it works on so many different levels (and subgenres) of horror.
It's part monster horror, with a monster that is absolutely terrifying in every form and shape it takes.
It's part psychological horror, where you really feel the paranoia of the characters and understand them going crazy and it plays into the madness of the monster.
It's part suspense horror, with at its heart the 'whodunit' (or better 'who is it?') scenes where it just builds and builds because the thing absolutely refuses to reveal itself until it is discovered.
And of course gore hounds get their fair share as well, with blood and guts everywhere each time the thing strikes.
The Shining is really the only other horror movie I rank so highly for pulling off such a flawless and effortless combination of different horror elements.