X-Men 3
So this was not a bad movie, as I’d been misled to believe. It was, in fact, a perfectly fine movie easily on par with the first one. (Tangent: There’s been some weird revisionism where people are turning the first movie into this awesomely great thing in retrospect. It never was awesomely great. It only seemed verging-on-great at the time because there were no good superhero pictures in the history of ever (with the maybe possible exception of those Superman movies from the ’80s), so everyone was surprised. But by the superhero-saturated standards of today, it was very just okay — better than Daredevil, not nearly as good as Spider-Man. Fantastic Four-esque, maybe.)
The biggest mistake it made, honestly, was trying to do the Phoenix storyline at all. That’s one of those “cosmic” storylines that only makes sense if you’re willing to have galaxy-spanning adventures, plus also requires a ton of time to do right (since it requires a slower change with more foreboding and shit). In the realist aesthetic that pervades modern comic book movies, it’s just impossible to do satisfactorily, so it ends up being some weird side-point to the main plot, tacked on just because everyone figured it kind of needed to be.