I'd say all of them made too many sequels. After awhile, it's the same soup warmed over.
"Friday the 13th" was interesting only because you could never figure out who (or what) Jason actually was. In the first movie, there wasn't even a Jason doing it killing; it was his Mother. Later on, he seemed to be a supernatural being.....but how did he get that way? At any rate, all he did was find imaginative ways to kill young people at Camp Crystal Lake. Later, he started killing them in Manhattan, and later still, he moved to outer space, for gosh sakes. Meh.
"Nightmare on Elm Street" was unique in that it wasn't all just senseless murder; there were a lot of situational aspects to Freddie's deeds; a lot of bait-and-switch took place instead of just wholesale slaughter. Still, the idea of a supernatural killer who wisecracked his way through a movie made me turn away after awhile.
If I had to pick a favorite, it would be the "Halloween" movies, due to the actors involved. I thought Jamie Lee Curtis was amazing in the first two segments, Danielle Harris made you want to jump in and help her during the 4th and 5th movies, and Donald Pleasance was an earnest monster-slayer throughout the whole series. Each chapter was different in it's psychological aspects; this was so because unlike Jason and Freddie, Michael Myers wasn't the focus of the movie....the people fighting him were.