1. Superman IV: The Quest For Peace
Some of the movies on this list were "so bad, they're good", while others were just pretty poor watches. None, however, produce the overwhelming feelings of disdain that Superman IV: The Quest For Peace does.
The film's predecessor Superman III had its issues, but it looked like it belonged to the mighty Superman franchise. This one didn't even look like a Hollywood movie, so that - and the fact that it managed to double down on the previous movie's issues - made it a much, much harder watch.
I don't even know where to begin with this one. The special effects are so poor that it's crazy to think they ever made it to the screen. It's not a case of just being considerably worse than the visuals of the original 1978 movie (that was released nine years prior), it's that they are horrendous and not at all worthy of the franchise they appear in. And the material certainly isn't worthy of the actors involved.
The great Christopher Reeve was dissatisfied with the movie during production as well as the final product, which ended up being nothing like anyone who cared about the franchise wanted it to be. It was a shell of what the Superman franchise represented and a sad conclusion for one of the most iconic film series in the history of Hollywood.
Everyone involved deserved much, much better.