Certainly not the first slasher movie, but arguably the most influential.
This original film stands alongside Part V: A New Beginning as the only two movies in a 12 film long franchise where they play the killer as a mystery, I think that premise would wear thin had they tried it every go around, but here it is executed, if not masterfully, at least to the film's benefit. Friday the 13th is a movie I'll watch at least once a year, ever since I first saw it in the early 2000s. Maybe it's not the best movie ever, or even the best slasher movie ever, Hell, maybe it's not even the best Friday the 13th movie ever, but the fact that I'm not sick of it yet has to count for something.
Final rating:★★★½ - I really liked it. Would strongly recommend you give it your time.