Comic books, whether in the North American mainstream or Japanese manga, have no shortage of overpowered superheroes saving the day. One of the most powerful in the landmark DC Comics world of Watchmen is the omnipotent Doctor Manhattan, a being who exists outside of time itself while controlling the very fabric of reality. One of the strongest superheroes in the world of manga and anime is Saitama, the unassuming protagonist of the acclaimed One-Punch Man franchise.
Manhattan was once Jon Osterman, an atomic scientist who is caught in an experimental generator and has his body reconstructed completely. Detached from humanity, Doctor Manhattan can see all points in time simultaneously, is nigh-invulnerable, telepathic and can manipulate matter around him at all, including creating multiple copies of himself at once. The character's growing omnipotence and unique perspective on time and space have led him to grow distant from others and his own heroism, often opting not to intervene when he could easily change events in both big and small ways.
Saitama was an ordinary man who stopped a monstrous supervillain named Crablante while on the way to a job interview. Inspired, Saitama began a daily training regimen of performing 100 push-ups, 100 sit-ups, 100 squats and running 10 kilometers every day. The training routine led Saitama to lose all his hair and defining facial features but made him virtually indestructible and capable of feats of immense strength and speed. True to the property's title and premise, Saitama can dispatch any opponent he faces with a single blow while withstanding massive attacks without suffering even a scratch. This pushes him to search for the one opponent who can endure more than one hit to make his superhero life a little more interesting.
So, what would happen if these two immeasurably powerful individuals came to blows?
Manhattan can and has rewritten all of reality around himself based on his views, completely altering the DC Universe and the characters within it. With this in mind, Manhattan could simply just remove Saitama from existence in the space-time continuum entirely. Manhattan's unique relationship to time could also have him travel back to a point in history before Saitama began his rigorous training regimen and attack him at a significantly more vulnerable point in his own history. And while a punch from Saitama would likely obliterate his physical form, Manhattan could reconstruct himself completely.
The only real chance Saitama may have is if Manhattan's detachment from Earth, his own humanity and his overriding resignation of fate leads him to simply accept a potential defeat at Saitama's hands. Manhattan can foresee any attempt by Saitama to destroy him and may simply concede that his destruction is something of an inevitability, allowing the punch to land at full force and not see any discernible merit in reconstructing himself. This would indicate that Manhattan would deliberately throw a fight, similar to allowing Superman to nearly land a punch in Doomsday Clock or accept Ozymandias' scheme in the original Watchmen story.
While Saitama could probably match Doctor Manhattan punch-for-punch, Manhattan was never really a traditionally physical type of superhero. It is Manhattan's dominion over time, space and matter itself that gives him the edge over the unassuming manga superhero. However, Manhattan also has been increasingly less interested in contests of strength and his own role in the world, so if Saitama caught him on a day of particular existential detachment, Jon Osterman could be another figure defeated by the Japanese superhero, should he decide to concede entirely.
In a hypothetical fight, Manhattan could defeat Saitama if he tried but the omnipotent figure has been trying less and less as time has gone on. And that could make all the difference.