X-Men is one of the biggest examples of "My Real Daddy", where a work becomes more popular and defined by a different creator than the originator. It's definitely true that Chris Claremont was biggest mastermind behind X-Men we know now (alongside others like Len Wein, David Cockrum, John Byrne, and so on). He may not have created the X-Men themselves, but he did create many, many characters and worldbuilding elements associated with the team, such as Rogue, Psylocke, Shadowcat, Phoenix Force, the Brood, Lockheed, the Shi'ar, the New Mutants (Magik, Sunspot, Cannonball, Karma, Cypher, Warlock, Magma, Wolfsbane, Dani Moonstar), Excalibur, Madelyne Pryor, Mr. Sinister, Gambit, the Hellfire Club, Emma Frost, Strong Guy, Rachel Summers, Forge, Mystique, and so on.

I also notice that X-Men is probably the Marvel property least associated with The Man and The King, compared to the likes of Fantastic Four, the Avengers, Spider-Man (for Stan), Captain America (for Jack), and the general MU. It's actually rather easy to forget Stan and Jack were technically the creators of X-Men, and they were the ones who created iconic characters Cyclops, Angel, Beast, Iceman, Jean Grey, Professor X, and Magneto, as well as coming up with the mutant concept in general, originally to avoid having to come up with intricate backstories of how they got their powers, but also to commentate on real world social issues without the baggage of the real thing.

In my opinion, you can't really say they aren't the creators, even if the X-Men as a whole became radically different, more expansive, better defined, and much, much more popular without them. At the end of the day, they were still the ones who put pen to paper and conceptualized the X-Men to begin with. Yes, it was met with lukewarm reception, middling sales, with both men leaving relatively early in the run, and was seen as a poor man's Fantastic Four that ultimately got cancelled for five years before coming back better, but they still provided the foundation of for later creators to work with. Put simply, it's like denying that Jerry Siegel and Joe Shuster created Superman or that Bob Kane and Bill Finger created Batman, even though they themselves changed radically over the decades that followed.

It's also a counterargument I refer to when someone tries to suggest the X-Men are somehow "less Marvel" than the others and should be separated into their own universe. That of course being "The X-Men were created by Stan Lee and Jack Kirby. You don't get more Marvel than that".

So that being said, how much credit do Stan and Jack deserve for X-Men overall?