Mjolnir, the hammer of Thor, is one of the most powerful artifacts in the Marvel Universe, with the ancient weapon only usable by those deemed worthy enough to withstand such power without corruption. In the Marvel Universe, there are only a handful of people who are worthy of lifting the hammer. And in the DC Universe, there are actually two notable heroes from the DC Universe who have claimed the power of Thor's hammer for themselves when their worlds crossed over.
Across two massive crossovers, Superman and Wonder Woman both got the chance to wield Thor's hammer, Mjolnir. However, only one of them was truly worthy of Mjolnir's Asgardian power.
WONDER WOMAN AND THOR'S HAMMER
At the heart of 1996's DC Versus Marvel by Peter David, Ron Marz, Claudio Castellini and Dan Jurgens, a number of figures from both universes are pitted against one another for the potential fate of their universes. One of these duels ends up being between Thor and Shazam. Although Thor is able to win the bout by disrupting the magical lightning that empowers Billy Batson, it results in Mjolnir being blasted away. Shortly after that, Wonder Woman reads the inscription upon the hammer and takes it up, which grants her the power of Thor, on top of her already impressive abilities. Thanks to her noble spirit, immense strength and role as a potential Queen of the Amazons, Mjolnir quickly decides she's more than worthy enough to wield the hammer.
But that worthiness also ends up being Diana's undoing in the tournament. Pitted against Storm from the X-Men, Wonder Woman decides it would be an unfair advantage to engage the mutant with the additional power boost that comes with Thor's hammer. Wonder Woman ends up losing this fight, bombarded by repeated lighting barrages from a reluctant Storm that eventually overwhelm Wonder Woman. Later, many of the heroes of the two worlds would come together to try and break up the chaotic brawl that erupted between Darkseid and Thanos. During the fight, Thor and Wonder Woman end up next to each other, and Diana easily hands the God of Thunder his hammer, catching Thor by surprise as the battle rages on around them.
SUPERMAN AND THOR'S HAMMER
After coming across the Marvel Universe in 2004's JLA/Avengers by Kurt Busiek and George Pérez, Kronan is confronted by the Grandmaster, who arranges for a challenge to save his universe. This pits the Justice League against the Avengers, with many fights breaking out over the course of a galactic hunt for items from both universes. At one point, Thor and Superman end up coming to blows, with the Man of Steel barely winning out, even proving capable of catching the hammer in his hand and finally knocking out Thor with a punch to the face. However, this battle weakened him enough for the rest of the assembled Avengers to pile onto the Kryptonian. After the true scope of the battle was revealed, the two teams put aside their differences and worked together to stop Krona's actions from destroying reality.
Heading into the final battle, Captain America remains behind to serve as a field commander, giving Superman his shield so he can lead the charge against Kronan. The battle takes many dark turns, until Superman is the only hero left who makes it to Kronan's last line of defense. Unable to breach the wall, an overwhelmed Thor throws Superman Mjolnir, and he's able to use the hammer to finally break down the cosmically powered barrier. Afterward, however, Superman is shown to not be worthy enough to lift the hammer, with Thor explaining that an observing Odin made an exception for Superman due to the circumstances. This implies that while Superman is technically worthy enough to lift the hammer in a pinch, he's not nearly as worthy as Wonder Woman, who lifted the hammer herself with no additional assistance.