The universes of both My Hero Academia and One-Punch Man depict superhuman societies protected by a multitude of heroes with varying power levels. The heroes of One-Punch Man are generally thought to be more powerful than the ones of My Hero Academia due to their more impressive feats of strength, speed and durability. However, there's one hero in My Hero Academia who stands heads and shoulders above the rest of the pack -- the one that every other hero in the series compares themselves to and strives to be like: All Might.
The heroes of One-Punch Man are divided into four classes: C, B, A and the elite S-Class based on their abilities and power. What rank would My Hero Academia's number one hero All Might, in his prime and at the start of the series, fall into if he worked as a hero on Saitama's Earth?
What Is All Might's Power Level?
The monsters and heroes of One-Punch Man move faster and hit much harder than most of the heroes of My Hero Academia, with the top of S-Class being able to level buildings with a single hit and move at hypersonic speeds invisible to the normal human eye. The other top heroes of My Hero Academia, such as the flame hero Endeavor, are much more human in their feats and accomplishments. However, All Might is so above his peers that his strength and speed alone would comfortably put him into One-Punch Man's S-Class.
All Might during My Hero Academia has shown striking feats that would put him at the top of S-Class. In the show's first season, All Might is shown to be able to change the weather with a single punch -- an accomplishment only Saitama in One-Punch Man has been able to match. His hits are so powerful they can level entire city blocks, as seen with his United States of Smash. However, in terms of speed, All Might is greatly outclassed even by the lower-level S-Class heroes. The villain Speed O'Sound Sonic, Saitama's disciple and friend Genos and the ninja hero Flashy Flash have the ability to move and fight at multiple times the speed of sonic at the bare minimum. All Might's best speed feat comes from when he traveled 5 kilometers in under 30 seconds to reach his enemy All For One. This puts his top speeds at just above the speed of sound -- incomparable to the faster heroes in S-Class.
All Might's punching power may put him toward the top of S-Class, being far above One-Punch Man's striker heroes like Superalloy Darkshine and Puri-Puri Prisoner. He may also find himself having trouble reacting to and keeping up with the faster monsters that characters like Flashy Flash and Garou could have no trouble matching. However, All Might in his prime was at a different power level altogether. In his fight against the genetically modified Nomu in My Hero Academia Season 1, All Might claimed that it would've only taken five hits to defeat an opponent he was forced to punch over 300 times. On occasion, All For One and Shigaraki have also commented on All Might's loss in speed and quickness compared to his prime. So, if prime All Might was placed into the One-Punch Man universe, his speed, strength and durability may have been more comparable to the top five of S-Class among fighters such as Bang and Atomic Samurai.
All Might's Shining Personality
The Hero Associations of One-Punch Man and My Hero Academia are similar in that they rank heroes not just based on combat ability but also popularity. In that aspect, it's almost undeniable that All Might clears every S-Class hero. All Might is a symbol of peace who tirelessly fights villains and puts his life on the line to create a safer society. His smile and charisma comes across on television and inspired an entire generation of heroes who looked up to him.
In comparison, the majority of One-Punch Man's S-Class heroes keep to themselves and have little regard for the optics of hero work. Heroes like Drive Knight and Metal Knight never make public appearances and work from the shadows, while other characters like the number two hero Terrible Tornado are unlikeable and show resentment toward weaker humans. All Might would be the most popular hero in the One-Punch Man universe, and that quality alone would help keep him toward the top of the rankings.
It's difficult to compare combat feats between universes like One-Punch Man and My Hero Academia. However, what audiences have seen of All Might's strength and personality would put him comfortably in the top ten of S-Class as they see him at the start of the series. At the peak of his power, All Might would likely rank as high as third on the rankings, just behind Terrible Tornado and Blast.