Obviously there can be no definitive answer, but in addition to the many shockingly awful movies here, I’d nominate Fat Guy Goes Nutzoid. I swear, the only possible explanation for the end of the movie is that they ran out of money and just had to cut it there.
I was watching some of the MST3K reboot (continuation?!?) on Netflix. Most of the movies were bad but just bad. There was one, though, Cry Wilderness, that is crazy terrible.