But I don't think that any of Glass's symphonies could qualify as the thread topic...
No, because for all their thinness of content, they are well-written. For a truly "worst" piece, you need a composer who aims high, but much higher than he can achieve. The sort of thing Mozart satirises in his Musical Joke.
There is a lovely book of verse called The Stuffed Owl, with sections on "the bad poet at his best" and "the good poet at his worst" ("Across the wires the electric message came:/'He is no better. He is much the same.'" - Austin). Probably the truly worst music is the bad composer at his worst, but you will never hear that. The fun is finding the good composer at his worst - Harris 13, anyone?
A university student some years ago turned in a composition entitled "Parabolas", written in such a way that the notes on the page made a series of parabolas. It was not performed ...