Halloween to Scream to It Follows. The horror sub-genre that produced the highest-volume body count and the lowest critical reputation, but at its best has earned both.
Slasher horror is the most-formula-driven sub-genre of the horror tradition. The structure: a killer (usually masked, usually silent), a group of victims (usually teenage or young-adult, often punished structurally for various transgressions), a 'final girl' who survives. The conventions were established by Psycho (1960) and Halloween (1978) and have been repeated for fifty years.
The slasher's reliability as a commercial form is also its critical weakness. The conventions are so well-established that most entries are working from a template rather than developing the form. The result is that hundreds of slasher films get made; most of them are forgettable; the genre's reputation suffers from the volume of mediocre product more than from the quality of its best entries.
The films above are the entries that have, in our reading, earned the genre's seriousness rather than just confirming its template. The remaining ten thousand entries in the genre's history are, mostly, products rather than films.