Cyclops in X-men 3. The whole Dark Phoenix Saga has always been about Scott and Jean’s character development, their star-crossed romance, and Jean’s epic self sacrifice in the end.
You can’t just strip all that away just to make it into yet another Wolverine movie.
(Without getting too off topic, the Dark Phoenix story is too big for one movie, IMO. That joint needs to be a two parter… Simon Kinberg learned that lesson the hard way.)
Also, there’s Darwin in X-men first class. This might be the most
movie death in recent history. They built up a character with the power to adapt to anything just to kill him off.
I always found it pretty suspect that ALL of the non-white mutants in that movie either died or turned heel and joined Magneto in the end.