Curse of the Headless Horseman (1972) 
It's not often I think this (least of all say it), but Curse of the Headless Horseman is quite possibly one of the worst films I've ever seen.
It tells the tale of Mark, a hippie medical student who inherits his uncle's Wild West themed ranch theme park under the stipulation he can keep it if he turns a profit within the first six months.
However, the ranch is a dilapidated run-down mess so Mark decides to move in along with his hippie pals and set about drawing in some custom. Once there the group of hippie stoners are 'haunted' by a headless horseman who rides around the ranch splashing blood at people from his severed head collection.
This may well sound fairly entertaining, or even quite amusing, but the fact is that the themes here (hippies meets cowboys meets The Legend of Sleepy Hollow) are executed so poorly that the result is nothing more than an dull, plodding and amateurish rendition of Scooby Doo with terrible acting, bad dialogue, and unconvincing kills. A real chore to endure.
Originally posted here:
Nightmare USA Films Discussion Thread