OnePunch-Man is originally a webcomic by "One" which is currently getting a remake as a webmanga with drastically improved art. OPM is the story of Saitama a superhero by hobby which has a slight problem... he became too strong. No matter who he fights he always wins with one punch. Soon Genos appears, a cyborg seeking revenge on a robot that destroyed his home town and in his quest he encounters Saitama and becomes his disciple in order to become powerful enough to fulfill his mission.
Eventually the story expands introducing the heroes association which Saitama becomes a part of and beings that threaten the entire world's peace, and as usual Saitama must appear to stop them! Will there ever appear an enemy worthy of his time?
OnePunch-Man is a very strong verse. There are very few hax powers in the serie so far, but it is incredibly strong in the physical department.
The heroes and "monsters" are divided into different classes.
The heroes are divided into C, B, A and S class. C classes deal mostly with petty crimes. B classes with minor dangers, A classes with mid ranked threats and S classes usually deal only with demon class threats and above.
The "monsters" are divided into threat/disaster classes:
Wolf : potential threat, and threat to its immediate surroundings
Tiger: threat to a mass number of people
Demon: a threat for up to an entire city
Dragon: a threat which easily surpasses the boundary of a simple city (likely up to country lvl threats)
God: a threat to the entire world
Currently in the verse there have been registered at least q continent+ physical strength (likely much higher), lightning fast speeds, psychics strong enough to hurt dragon class threats and 2 god class threats, one of them being implied to be at least a life wiper. And all of this is still far below Saitama. Furthermore, rank 1 S class hero is yet to appear and there is an existence called "God" which can casually grant and take dragon lvl powers without even making a physical apparition. The top fighters of the serie are at least 4 digits in speed.