[knife_fight]

the Evil Dead series for me. I'm a big fan of the Exorcist, but the 2nd one and the prequel really blew it for the whole franchise.

But, the "Mario Bava series" is probably my favorite. I know that's not the kind of series we're talking about, but it does say "Mario Bava Series" at the top of all of those dvds. Bava, to me, is great. All of the movies in this "series" are, at the very least, worth watching... unlike all of the typical answers to this question (F13, NOES, Halloween, etc).