Many Marvel characters have used the incredibly powerful Infinity Stones to perform incredible feats, but Saitama from One-Punch Man may be able to use one of the stones better than any of them. Fans have often imagined how the incredibly strong Saitama would fare in a fight against Marvel’s heaviest hitters. While many of these hypothetical fights may never be fully settled, if Saitama had access to the Space Stone, there is little doubt that he would be unstoppable.
The Infinity Stones are six gems in the Marvel Universe that can each grant their wielder supreme control over a specific aspect of the universe. This includes natural forces like Space and Time, as well as specifically human elements like Mind and Soul. When combined together in the Infinity Gauntlet, each stone can fuel another stone, forming a loop that can grant infinite power. However, when separated, the stones’ power depends on the wielder’s proficiency in the specialty of another stone. So, the Power stone is powered by the user’s mastery of the Mind, the Mind stone is powered by the user’s mastery of Soul, etc. While this limitation means the gems only become all-powerful when locked into the repeating circuit of the Infinity Gauntlet, a true master of any of their key disciplines can wield the gem that runs on that discipline at full power.
Saitama is the protagonist of One-Punch Man, a manga written by ONE and with art by Yusuke Murata. He is an overwhelmingly powerful hero who can defeat any foe with just one punch. Saitama gained this immense power by undergoing a strict physical training regimen of his own design. Before doing this, he was just an ordinary man without powers. So not only is he incredibly powerful, but he earned all of that power through his rigorous hard work, meaning that he has incredible mastery over that power as well. As such, he would be able to use the Space Stone to its fullest capacity, as the Space Stone is powered by its user's mastery over Power. In every conceivable way, Saitama fits this description, to the extent that his big problem is being unable to find a worthy opponent who can take more than one of his world-shaking punches.
While Saitama is already incredibly fast, the Space Stone would enable him to instantly teleport. This would be very handy in a fight, letting him dodge the blows of powerful enemies. But the more useful application in Saitama's eyes would probably be the ability to travel anywhere in the universe. Saitama desperately wants to find a powerful opponent that he will need more than one punch to defeat, but time and time again the threats on Earth prove to be too weak. One of the most powerful threats he faced was Boros, an alien warlord who attempted to conquer Earth. This implies that in order to find more powerful enemies, Saitama has to venture off of his home planet, and the Space Stone would provide him the perfect means of doing so.
While there are Marvel characters powerful enough to alter reality, Saitama's pure physical strength seems to eclipse that of characters like Thanos and the Hulk and - crucially - he earned his power through his own effort, expertise, and commitment. Saitama has mastered power in every sense of the word, making him the perfect candidate to wield a cosmic artifact that increases in power in direct relation to that quality.
Ironically, while the Space Stone might let Saitama find more formidable opponents, it also gives him a major power-up, making it even harder to find someone who can match him in a fight. It is unlikely that One-Punch Man and Marvel will ever have an official crossover (but not impossible, given past partnerships with properties like Attack on Titan), so Saitama won't be able to actually obtain the stone. But if he ever did, One-Punch Man's protagonist would be the Space Stone's ultimate wielder, easily a match for any character in the Marvel Universe.