Black Christmas (1974)
Every time I see this I am more impressed with how brilliant a movie it is. Along with Halloween (1978), The Bird with the Crystal Plumage (1970), Suspiria (1977), and a handful of giallo, it's a genuinely great slasher film. Some of the shots of the sorority house are spectacular and in general all the Toronto film locations, including Trinity College, are artful and subtle.
Obviously almost all slasher films are trash. Even very well done movies like Scream, Nightmare on Elm Street, and Hellraiser get by with stylistic self-reference that's as important as the drama; they do not have any serious involvement with the characters. It's so unexpected to find a movie that is scary as hell but also just a very, very good serious movie too.