Manos - The Hands of Fate
and
The Beast of Yucca Flats
are probably the worst ones I ever saw. I actually like bad movies, they're usually funny, but those two shouldn't even be called movies, watching them just made me aggressive since nobody involved had any bit of talent or at least tried to do something more than the absolute minimum.
Manos was filmed with a camera that could only record 32 seconds of film, that explains the quality of the movie a bit. No, that's no joke.