Justice League Only Has One Essential Member (And It's Not Superman)
Though often seen as a pillar of the Justice League in DC Comics, Superman is actually the first person to admit another hero is the glue that holds the super team together. While the founding members of the League have changed throughout the years amid reboots and retcons, several heroes have generally had a stable presence on the team. Despite the Man of Steel often taking a leading role, even he knows that one other hero is the Justice League's most crucial member.
The Justice League first appeared in Brave and the Bold #28 by Gardner Fox, Mike Sekowsky, Joe Giella, Murphy Anderson, and Bernard Sachs, where the heroes Flash, Green Lantern, Wonder Woman, Aquaman and Martian Manhunter banded together to defeat the invading Starro. Since their initial adventure, the team has had a rotating cast of some of the biggest superheroes in the DC Universe. And while Superman is known for his stalwart leadership, by the hero's own admission, another Leaguer fills the role of heart of the team.
J'onn J'onzz, aka Martian Manhunter, has been with the Justice League since their earliest days. And according to Superman, there wouldn't be a League without him. In JLA #118 by Geoff Johns, Allan Heinberg, Chris Batista and Mark Farmer, the team discusses J'onn and if he'd be willing to use his telepathy to remove memories from the minds of their enemies. Superman already knows the answer their teammate would give, calling J'onn one of the most responsible and principled members of the Justice League. Clark recognizes that unlike most heroes, Martian Manhunter has been a part of more incarnations of the League than most, going so far as to call him the only thing holding the team together.
This sort of mentality isn't a one-off as Superman expressed a similar line of thinking in Martian Manhunter #0 from John Ostrander, Tom Mandrake, Carla Feeny, and Bill Oakley where Clark telling J'onn "You are the League--perhaps more than any of us." Superman isn't alone in this opinion, with Blue Beetle going so far as to call J'onn "the spinal cord" of the team in Countdown to Infinite Crisis #1 from Geoff Johns, Judd Winick, Greg Rucka, Rags Morales, Ivan Reis, Jesus Saiz, and Phil Jimenez. Clearly, the Manhunter from Mars has a stellar reputation among his colleagues, with many holding him up as the League's unsung hero.
Martian Manhunter has no doubt been one of the League's most underrated assets. Despite having strength comparable to Superman, he's often passed over in favor of the Big Blue Boy Scout. But these private admissions show that while not the most public-facing hero, J'onn serves a vital internal role. Other Leaguers see his constant presence on the team as a symbol of their overall stability. For as long as there's been a Justice League, J'onn has been there providing wisdom and guidance that other heroes, even Superman, rely on. While the Justice League is home to a number of outstanding heroes, Superman ensures that the right ones receive credit where due.