9. Dark Phoenix
Where did it all go wrong for the X-Men? A franchise that, once upon a time, was the biggest superhero film series of them all, its final official installment was unfortunately its darkest day.
The franchise had already tried adapting the Dark Phoenix storyline in X-Men: The Last Stand to very mixed results, so I don't know what possessed them to do it again in 2019, especially after previous installment (2016's X-Men: Apocalypse) wasn't all that well-received. But alas, Fox soldiered on with the story, adapting it with the younger cast this time... and producing a result far, far worse than the original adaptation.
Dark Phoenix is a grim, dull, and lifeless affair that swats away the triumphs of the X-Men saga in favor of heartless darkness. The clue is literally in the title (which dropped the "X-Men" for no reason), but not even that could prepare you for the absolute joyless misfire this one is. It's not that a more serious X-Men movie couldn't have worked (see: Logan), but the execution of Dark Phoenix left a lot to be desired.
The film helped no one, destroying a premise with much potential, doing a complete disservice to the cast, and making viewers cry at how far the franchise had fallen. We didn't need an adaptation of the Jean Grey Phoenix arc the first time, but there was a lot of story potential in the first attempt; the second threw all of that potential away.
This was not the note that the almighty X-Men franchise should have gone out on.