As a horror movie fan who has seen A LOT of bad ones, it would be far too easy to just pick a bad horror movie as the worst movie I've ever seen. So I'll just choose a regular mainstream Hollywood movie, and that one is Nothing But Trouble (1991). This is an excruciating comedy with a star-studded cast that includes Chevy Chase, Demi Moore, John Candy and Dan Akroyd. I only saw it once a long time ago, but I certainly remember it being beyond horrible and very painful to watch. A complete waste of good talent.
I guess having some mix-up nostalgia memories, due to often mistaking it for Beetlejuice (1987) because I never got to see either of the films, all the way through. But yeah, childhood and usually likeable names as John Candy and Dan Aykroyd, might help me to overlook certain things, but having just purchased and seen Haunted Honeymoon (1986) with Gene Wilder and Dom DeLuise, wow, that was bad, really awful "comedy", which at times reminded me of Nothing But Trouble. Probably seeing so many talents going to waste, as it was painfully unfunny, and such a letdown, because I really wanted to get more out of it, that and the 2022 DVD version, was very pricy.
Never did see Haunted Honeymoon and that's because it looked bad after seeing the TV ads.