Good question. There have been quite a few, but if I had to single out one... I'd probably have to go with Superman after Superman II. Superman III and IV are both awful and cheap-looking movies (the latter not even looking like a professional film) where it's clear that the producers didn't know or even care where to take the character. From there, Superman was in limbo and you had a lot of false starts on projects (Superman Lives, Ratner's Superman, Abrams' Superman) interspersed with some moderately successful TV shows (Lois & Clark, Superboy, Smallville). When Superman Returns finally came out in 2006, it was obvious that WB thought the only way to make the character work on the big screen again was just to copy what Richard Donner did (but instead of just remaking the film, they dressed it up as a sequel that basically follows the same plot beats) and the results were lackluster. So then they hit the reset button and it looked like they had a surefire winner since Chris Nolan was onboard. But they hired Zack Snyder to direct, and while I like MOS, I can't deny that the movie was divisive and rubbed a lot of fans and critics the wrong way. And so then you had BvS, which started out as a Superman sequel but ultimately became a Batman movie where Superman was reduced to background character. And next up with have Justice League, and we have no idea how prominently Supes will factor into it.
Such a disappointment. The original superhero deserves better than this. Now, don't misunderstand me here... there are certain posters on this forum who claim that people who feel the way I do "want" the DCEU to fail. That couldn't be farther from the truth. As angry as I may be with WB and Snyder over BvS, I desperately want JL to be good and for Superman to really shine in it. I want more solo Superman films. But will that happen? Right now, "maybe" is about the most hopeful answer I can give.