Only the truly worthy have ever been able to lift Thor's hammer, Mjolnir, but as every Superman fan will tell you, the Man of Steel proved himself worthy when Marvel and DC crossed universes. The problem is... that isn't really the entire story, at all. And the comic itself explains the sad truth behind Superman's 'worthiness.'
The question of another Marvel hero lifting Mjolnir returned to the spotlight thanks to The Avengers: Age of Ultron, showing the MCU's top challengers trying their best to lift the hammer. Captain America could only nudge the hammer back then, but proved himself worthy of Mjolnir in Avengers: Endgame. And when he did, comic fans couldn't help but mention that he wasn't first to wield both Captain America's star-spangled shield and Thor's hammer.
That honor belongs to Superman, who accomplished the task in the JLA/Avengers crossover event. But while the famous comic artwork might suggest Superman is as 'worthy' as Thor himself, the truth is nowhere near as glorious.
Yes, Superman Wields Mjolnir in Battle
The reading of the artwork may be one of the most often-repeated, but not-quite-accurate DC/Marvel battle moments, so fans will likely wish to know the truth. As much for their own curiosity as for the chance to correct those who put Superman on Thor's level of godliness (now along with the MCU's Steve Rogers). So let's start with the scene in question, as every Thor and Superman fan know it.
The epic JLA/Avengers crossover from Kurt Busiek and George Pérez holds up to this very day, with the villainous Krona merging the Earths belonging to Marvel and DC as part of a much larger, much more nefarious plot (which we won't dive too deeply into here). The heroes aren't as stunned as one might think by having their universes collide, since Marvel and DC confirm they share a Multiverse fairly often. So as the heroes attack the forces of Krona (and countless villains from both sides), the battle turns against them. Superman succeeds in pummeling his way through to one last barrier... only to find "the last wall, Krona's final defense! But it's energized" with a forcefield his raw strength can't beat.
Witnessing the last hurdle, Thor cries out for the Man of Steel's attention, and sends Mjolnir flying straight towards him. The result is the moment every fan already knows, with Superman holding Cap's shield on his left arm, and Thor's mighty hammer in his right. It's only made a more epic moment with Thor's command to "Do Thou Well With It!" followed immediately by Superman -- one of the strongest beings in the entire universe -- visibly shaken by the power coursing through his body. Superman has a weakness for magic, but in this case it results in a serious rush.
Using that magic of Asgard paired with his own worthiness (or so it seems), Superman smashes the final wall to pieces. A massive explosion later, and the reader can only watch along with the heroes as the crisis comes to a close. But the comic itself doesn't wast any time revealing the slightly disappointing truth behind Superman's most epic crossover moment. It is Thor's hammer, after all.
...But Superman Wasn't Actually Worthy of The Power
The massive explosion is followed by Hal Jordan (in his Spectre form) finishing up the crisis, leaving the battle-damaged heroes to commiserate. Realizing that they must make a speedy retreat to their respective worlds, Superman decides to return Mjolnir to its true master. And it's the scene which comes next that is almost always overlooked by fans of Superman and DC, since the Man of Steel goes to fetch the weapon for his new Asgardian ally... only to realize he can't lift it an inch. He is as confused as the reader -- not yet realizing that you can lift Mjolnir and not be worthy -- but the god of thunder thankfully explains the problem to both.
Thor explains that the enchantment placed upon the hammer means it needs more than muscle to be raised (and at certain points in Superman's comic history, he would actually be strong enough to raise an infinite mass hammer like Mjolnir). But how did he manage to wield it in the first place? According to Thor, "A very few worthies have been allowed to overcome the spell, in desperate hours." A close reading of the original scene shows both Busiek and Pérez may have been flirting with this explanation all along, knowing readers would assume Superman was just plain worthy.
But take a closer look, and Superman actually comments on his attempt to "maintain the momentum" of the hammer thrown by Thor. While the artwork from Pérez can be read as Superman simply redirecting the hammer into the wall. Superman doesn't actually use the weapon again, shown for just a brief second in the next panel, still holding it (hunched over) before it is blasted out of his reach. Certainly not anything close to the Endgame analogue, in which Captain America summons lightning like a true god of thunder.
Applying what Thor says before bidding Superman farewell, it can be deduced that the enchantment did not "grant Superman the power of Thor" at all. But it would allow a being to influence its path, if the times were desperate, and the figure attempting it worthy of that act. In a way, not all that different from Cap's wiggle of the weapon seen in Age of Ultron. The point is, those who hold up the image of Superman holding Mjolnir as evidence of his worthiness must also include the panels of Superman struggling to lift it, as Thor can only smirk at the truth.
Is it a slightly diminished moment for Superman? Maybe... but as Thor points out, it's his opinion that Superman would be capable of wielding it. But he doesn't make the rules. Oh, and for the record: Wonder Woman raising Mjolnir isn't undercut in the same way, she just chooses to drop the hammer out of a desire to fight fair. But that shouldn't surprise anyone.