A Nightmare On Elm Street cos It has the most rewatchable sequels. Only one I've got no interest in ever seeing again is Freddy's dead. Series could've kept going for a lot longer if the didn't increase the humour with each movie. They turned him into such a joke that by the time New Nightmare came out, nobody cared & it flopped. We would've got Freddy v Jason a lot sooner and a lot better had it been a success.
Scream is probably my all-time favourite franchise. It's so cheesy and not even particularly scary, but everything about it is just so good. Wes Craven >>>>>
They also did a great job at translating Scream into a TV series.
Honourable mention to Alien as well, the original is a legendary film that will go down in history.
so many of those mentioned have GREAT original Movies but bad/abysmal follow-ups. so since this is a question about franchises, i say:
*paranormal activity (first 4 are really good)
*Saw
*other (conjuring. both Movies so far are Amazing).
edit: forgot about Alien. that too! and the first too "living dead"-Movies are awesome aswell
my fav horror Movies however are the Shining, Exorcist, Psycho, texas chainsaw massacre and Saw.