Despite both being superhero teams working towards the common good of people everywhere, the X-Men and the Avengers often seem like polar opposites. The Marvel Universe adores one and hates the other. Since the launch of the Marvel Cinematic Universe, the Avengers have been the premiere super-team both in real life and within the Marvel Universe... but despite gaining quite a bit of popularity in the real world, they still lack the edge and cool factor that comes from being a team of outcasts like the X-Men. Still, despite that perception, both teams wind up sharing roster members quite frequently. Besides the Fantastic Four, they're the two teams with the longest tenure in the Marvel Universe, and... it's not like the Fantastic Four are exactly hiring once they reach their name-mandated quartet.
Where else are do-gooders in costumes and with a need to work in teams going to go? The Defenders -- the team that's not a team? Every other group is liable to fold sooner rather than later, and presumably hopping from team to team gets frustrating after a while. That said, just because some of these heroes tend to swap between the X-Men and Avengers a lot doesn't mean it's the best idea. The truth is, quite a few members of the X-Men could stand to stick with the Avengers for the sake of their own personal growth rather than going to their old ways. Similarly, some X-Men simply represent Xavier's dream too much to ever devote themselves entirely to Earth's Mightiest Heroes.
20 X-MEN: SABRETOOTH
After Marvel wrote off their signature berserker in 2014’s Death of Wolverine, multiple other characters stepped up in an attempt to take his place. In the aftermath of "AXIS", a Sabretooth who had his personality inverted tried to step into the role while he was still more of a hero.
He joined the Avengers Unity Squad in Rick Remender and Daniel Acuna’s Uncanny Avengers vol. 2 #1, but it didn’t last terribly long. Honestly the Avengers were a horrible choice for Sabretooth -- he spent most of his career as a vicious criminal, and only became a good guy after undergoing magic alteration. He barely fits on the X-Men, but he’s much less of a PR disaster there.
19 AVENGERS: PRODIGY
Prodigy became a member of the Young Avengers during Kieron Gillen and Jamie McKelvie’s time on the team. He actually lost his mutant powers ages ago though in the aftermath of M-Day, though he retains all of the memories and abilities he gained while working with the X-Men (probably for the best, since an alternate timeline showed if he’d kept his powers he’d eventually become a supervillain).
Now there’s not a key member of the X-Men who would reject him for no longer being a mutant, but that does give the character an excuse to spread his wings. He was already seeing differently from half the community during "Schism", why not let him serve as an Avenger, potentially even in a leadership role?
18 X-MEN: SUNFIRE
Considering Sunfire was a member of the second incarnation of the X-Men thanks to Len Wein and Dave Cockrum placing him in Giant Size X-Men #1, the character doesn’t get nearly enough respect. He joined up with the Avengers Unity Squad during Rick Remender and John Cassaday’s time on the title, but even that didn’t last particularly long.
For someone who was a part of one of the most popular versions of the most popular superhero team, he deserves to finally go to that next level. But if there’s any rehabbing that should occur with the character, it should happen with the X-Men.
17 AVENGERS: DEADPOOL
Deadpool’s been a member of both the X-Men (though briefly) and the assassination-focused X-Force before. And while it doesn’t make any sense, he was recruited by Captain America to join the Avengers Unity Squadron for a time.
While Wade’s a total loose cannon and probably the only anti-hero who’s got less respect for human life than most villains, one thing that’s been established is how much he’s willing to change in order to be accepted by people he respects. Both Spider-Man and Captain America have proven capable of bringing out a better side of him, and for that alone Deadpool is better as an Avenger.
16 X-MEN: STORM
Storm joined the Avengers during the latter end of Brian Michael Bendis’ run on the Avengers, with Avengers #19 in 2011. Recommended directly by her husband at the time Black Panther, she joined up to help deal with Norman Osborn and the remnants of H.A.M.M.E.R.
Her time with the team wasn’t long at all though, as she was forced to side with the X-Men during their conflict with the Avengers. Ultimately, that’s the crux of the problem -- Storm has been an X-Man for so long, not just as a member but as a key leader of the group, her true loyalties will always lie with them first.
15 AVENGERS: HAVOK
It’s not easy being a Summers. If you don’t wind up with absurd power or come from some future where mutants are nearly extinct, you end up in the shadow of Scott, founding member of the X-Men and one of the best leaders the team’s ever seen.
Havok spent most of his life trying to get out from under Scott’s shadow, and the only time he saw any success was when he was inverted thanks to "AXIS" and became a mutant supremacist. His time in the Avengers was a bit rocky, but he was still carving his own path both as a man and a superhero, and he should get another shot at that.
14 X-MEN: QUENTIN QUIRE
Quentin Quire is currently a member of Kate Bishop’s West Coast Avengers, a group of misfits getting up to hilarious hijinx out in California. He fits there well enough, but an official Avengers team? No thanks. His introduction came with him attempting to take over Xavier’s School, and since then he’s kept a reputation as being a giant pain to everyone he’s around.
With the amount of power he possesses and the fact that he’s literally housed the Phoenix Force before, and even kept a portion of it inside of him for quite sometime, he’s got a lot of growing up to do still, and it’s best it happens on the X-Men.
13 AVENGERS: CAPTAIN MARVEL
Carol Danvers was an honorary member of the X-Men during Chris Claremont’s run, going out into space and learning the ropes with her Binary powers with them. She was even offered membership to join as an official member, but she declined. As cool as having Carol on the team would, her relationship with Rogue won’t ever be… proper normal considering Rogue’s the reason she was taken out of action for awhile.
And while the X-Men could certainly use her abilities fighting their own battles, Carol’s much better suited to a position with the Avengers, tackling impossible adventures like battling Celestials aiming to destroy the Earth.
12 X-MEN: NAMOR
One of the oldest mutants around, the prince of the seven seas has served both on the Avengers and X-Men in different eras of his life. Specifically, he joined the X-Men in 2009 during their Utopia era, and that highlights why he’s better as an X-Man than an Avenger.
For all Namor’s attempts to help save the world, he always winds up going back to war with “surfacers” over some perceived slight. He’s a liability to the Avengers, and narratively it’s more interesting seeing him be a man trying to support two different groups of people constantly dealing with attacks from outside forces.
11 AVENGERS: FIRESTAR
If anything, Angelica’s best known for her time working with Spider-Man and Iceman during the '80s cartoon series. But the character in the comics has bounced around from Emma Frost’s Hellions to hanging out with the New Mutants, to being a founding member of the New Warriors before finally joining the Avengers after their return from the pocket dimension after "Heroes Reborn".
Currently she works with the X-Men as more of a teacher, but honestly she should get the chance to go back to the Avengers and prove herself without the shadow of her ex-husband Justice. With both the powerset and the experience, she’s a heavy hitter waiting to be given the proper acknowledgement.
10 X-MEN: CABLE
Cable joined up with the Avengers Unity Squad in Uncanny Avengers vol. 3 #3 by Gerry Duggan and Ryan Stegman, helping them deal with a handful of threats before eventually dropping off the team. Cable’s paramilitary nature doesn’t fit the way the Avengers handle things mostly, and his methods just barely fit the X-Men themselves.
There’s a reason he spends most of his time working separately from the team. That said, someone constantly working to stop horrible alternate futures where mutants are extinct should probably spend most of his time working with the team who already deal with people who hate mutants in the present.
9 AVENGERS: SCARLET WITCH
Rather than being an X-Man, Scarlet Witch spent most of her time early on as a member of the Brotherhood of Evil Mutants, joining the Avengers alongside her brother very early on in the team’s history. In any case, as a central reason for much of the anguish the mutants have suffered for 15 years or so, Scarlet Witch is probably one of the few people who couldn’t be recruited to the X-Men.
Especially in the wake of her retconned origins -- it’s hard to call yourself a friend to mutants everywhere while recruiting someone who eliminated the mutant powers of nearly every mutant on the planet.
8 X-MEN: X-23
Laura Kinney joined the X-Men in the aftermath of "Decimation", working with some of the few younger mutants who retained their powers for years before joining the Avengers Academy in 2012. She worked with the young heroes right up until they made it out of Arcade’s Murderworld during Avengers Arena before finding her way back to the X-Men.
While Laura has what it takes to be a member of the Avengers, it also feels like she has a bit of growing to do first with respect to her role as Wolverine, and the best place for her to do that is with her support system in the X-Men, especially now that she's got a little sister to raise.
7 AVENGERS: QUICKSILVER
Quicksilver has been a member of the Avengers longer than nearly anyone else, coming in during the team’s “Kooky Quartet” combination. The speedster was always a source of conflict in the group, but he was also never more of a hero than when he was working alongside the Avengers.
True, being around Scarlet Witch can make him more overprotective, but there are like a billion different Avengers teams -- put them on different ones. Plus, it’s better to have him here than anywhere near Magneto, even if the current continuity does suggest they aren’t related. Pietro needs as few negative influences as possible.
6 X-MEN: ROGUE
Rogue became a member of the Avengers Unity Squad that was formed during the aftermath of the “Avengers vs. X-Men” conflict, even going on to lead the team for quite some time. And while having Rogue pushed out of her comfort zone, taking on larger threats and leading a team was pretty cool, ultimately she’s just too much like a signature member of the X-Men to leave for extended periods of time.
Rogue is the X-Men, in the same way Cyclops and Wolverine and Storm are. Plus now she’s married to Gambit, who probably shouldn’t be allowed anywhere near the Avengers’ valuables.
5 AVENGERS: SUNSPOT
When Bobby was invited to join the Avengers, who knew he’d take to it so well? The cocky mutant didn’t just become a leader of men and women, he managed to subvert Advanced Idea Mechanics, turning it into an unofficial arm of the Avengers (and renaming it Avengers Idea Mechanics) during the run up to "Secret Wars".
He excelled in everything asked of him, even when the mutants were threatened with death thanks to the Inhuman cloud. The end of U.S. Avengers hinted he’d be returning to the X-Men, but what can he do there he wasn’t already doing as a part of the Avengers? Honestly, given how often Stark goes broke, the Avengers could use more than one financier.
4 X-MEN: WOLVERINE
As fun as it is to see Wolverine’s growth from mindless assassin to one of the people who’s fought the hardest for Xavier’s dream, being an Avenger takes it just a step too far. Brian Bendis’ ethos for having the character join the Avengers made sense at the time -- take the most important characters and place them on the biggest team to make the team itself feel big and important.
It worked for a time, but a decade into the Marvel Cinematic Universe the Avengers are a household name now. And let's be honest: how many times does Wolverine need to be on anyway? He never quits the X-Men, and winds up being on multiple separate Avengers squads -- where does he find the time?
3 AVENGERS: CANNONBALL
Cannonball was invited to join the Avengers alongside his buddy Robert da Costa during Jonathan Hickman’s run. He was a part of the expanded Avengers team, helping to defend against the threat which would eventually lead to "Secret Wars". He experienced some of his best growth here, being taken as more than just a member of the third generation of X-Men -- fighting battles in space, getting married, even having a kid.
This was a promotion for the character in every way, and he shouldn’t go backwards. Plus and Bobby were easily some of the funniest parts of Avengers World, and superhero comics don’t have nearly enough levity now.
2 X-MEN: DAZZLER
Dazzler was a member of the A-Force for a decent period of time, the all-women offshoot of the Avengers (as well as declining membership to the proper team). Alison’s been everything from a disco pop artist and rock star to an X-Men, Avenger, and even Agent of S.H.I.E.L.D.
But it seems more like she belongs with the X-Men over every other team, just from how she fits in with the group. She’s young and cool and an outcast -- everything the Earth’s Mightiest Heroes simply aren’t. While it’s cool to have “Former Avengers member” on her resume, ultimately she’s out classed in terms of the sheer raw power it takes to be an Avenger these days.
1 AVENGERS: BEAST
Beast is an unfortunate side effect of fans and creators alike being too attached to the way things are. Hank McCoy was allowed to grow the most when he was on the Avengers, working alongside his buddy Simon Williams/Wonder Man. He could be more than just the team genius -- he could crack jokes and have fun and not constantly think about how the world hated and feared him.
Plus he spent too much time with the X-Men and nearly broke time itself bringing himself and all the other original X-Men into the present. Even his brief stint as a member of the Avengers Unity Squad restored him to his old self and felt like a needed change.