When I think of the worst horror franchises, to me that means a franchise consisting of at least two movies in which EVERY movie has been rotten. So while a series like Sleepaway Camp has a couple awful sequels, at least the first flick was done moderately well so you can't really call the entire series bad. Same for Puppet Master, Hellraiser and some others mentioned.
That in mind, I'd say there's not a single good film in any of the following series IMO:
Jack Frost series
Bloody Murder Series
Scarecrow Slayer series
Paranormal Activity series
Those are the only ones which immediately come to mind, though I'm sure I could rattle off some more if pressed.