Unless you're counting "fascinatingly inept" as "bad", I disagree. The worst they did were the ones so incoherent and tedious that you couldn't sit through it no matter how funny they were.
The one that sticks out in my mind is Girl in the Gold Boots.
See now, I enjoy watching Girl in Gold Boots, but one that I can't wrap my mind around was the Wild World of Batwoman. At the end I realized I could recall a single thing that I had seen in the movie, it was like missing time or something
Thing with MST, though, is that "worst" is pretty much interchangeable with "best." Monster A Go Go in incredible in that it is such an anti-movie, and then throws in the deliciousness of that bizarre ending that genuinely lends an extra something to it.
For those unaware, the movie basically came about because H.G. Lewis needed a second movie for a double feature with another one of his movies, and so he took the reels for this one (which was unfinished by infamous B-movie crap-master Bill Rebane) and just edited it together with some new actors mixed in and narration to fill in the gaps, and back then that kind of thing would work and make a profit on a double bill of horror movies.
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