The Monster Association Arc of One Punch Man is undoubtedly the most impactful in the manga series.. It featured many powerful characters on the sides of both the heroes and the monsters, along with multiple fascinating battles. Though this part of the series is yet to be animated, the manga is greatly aided by the stellar quality of Yusuke Murata’s art, which makes it well worth reading.
Characters in One Punch Man are usually classified by hero rankings or threat level, giving viewers a good idea of the amount of power they possess. However, this arc introduces multiple new beings, each seemingly more powerful than the last. It also features evolutions and fusions that combine the powers of already formidable foes.
10 Genos
Genos is a cyborg well known by fans as he was introduced very early in the series and usually stays in close proximity with the series’ main character. He considers himself Saitama’s disciple despite placing higher than his master on the hero rankings. Being a cyborg, he has received multiple upgrades throughout the series, and this was also the case during the Monster Association Arc.
This time, his inorganic parts took on a full black color with a very intimidating appearance. Its focus was mainly on attack power, and its full strength was shown during Genos’ confrontation with the fusion of Psykos and Orochi. He was able to deflect one of the monsters’ energy attacks, and with his last resort attack—though at the cost of a part of his armor—he could completely offset it.
9 Platinum Sperm
Platinum Sperm is a being that arose due to a fusion of Black Sperm and another Mysterious Being. He defeated Superalloy Darkshine with no difficulty at all and proceeded to engage in battle with both Garou and S-Class rank 13 hero Flashy Flash.
Despite being impeded by Garou on multiple occasions, he matched Flashy Flash’s speed and knocked out the hero while mocking his ideals. He then traded blows with Garou and kept up with his astounding speed for an extended period of time. He also showed immense strength in using the sole tentacle protruding from his head to knock back other S-Class heroes like Atomic Samurai, Zombieman, and Puripuri Prisoner.
8 Sage Centipede
Sage Centipede is a monster that emerged suddenly after the battle between Platinum Sperm and Garou. It claimed to be the incarnation of father earth and was even larger and more powerful than the dragon-level threat elder centipede. Furthermore, it had regenerative capabilities facilitated by the core located inside its body.
It fought against both an evolved Garou and the S-Class rank 15 hero Metal Bat simultaneously and dealt significant damage to the two. Despite its great strength and pride, it was smart enough to recognize the threat its opponents posed and resorted to taking hostages to ensure its victory. It was only defeated after Garou knocked out its core and lured it into space, consequently splitting it in half.
7 Evil Ocean Water
Evil Ocean Water emerged shortly after Sage Centipede and was referred to by the latter as the “incarnation of mother sea.” It was originally known as Evil Natural Water, a being created by the Monster Association, but after its evolution, it grew much larger and much more dangerous.
Its massive size was facilitated by its ability to absorb other liquids and augment its physical makeup by doing so. It could also sense any hostility aimed towards it and react appropriately to each situation. Furthermore, its seemingly most potent attack was called Ocean Grand Cannon, and it was powerful enough to wound an evolved Garou.
6 Orochi & Psykos
The two monsters Orochi, and Psykos, were important figures crucial to the Monster Association. They were defeated in their regular forms by Saitama and Tatsumaki, respectively, but after merging together as one, they became significantly more powerful.
The fusion gained enough power to fight on equal grounds with Tatsumaki, and it was revealed that Psykos received additional strength from a being she considered to be God. It was capable of firing energy blasts strong enough to scrape off a significant part of the earth's crust and could stun Tatsumaki hard enough to make her temporarily lose control of her powers. Additionally, it was able to absorb the powers of others and use that to increase its own potency.
5 Tatsumaki
Fittingly dubbed as the Tornado of Terror, Tatsumaki is an astoundingly powerful esper who strikes fear in many who witness her power firsthand. She is the S-Class rank 2 hero and arguably the most powerful psychic in the series. She contributed massively to the protection of Z-City during the Alien Conquerors Arc and took on a more active role in the Monster Association Arc.
She fought toe-to-toe with the fusion of Psykos and Orochi, a being blessed by God, and dealt significant damage to it throughout their battle. In situations where brute force wasn’t enough, she explored different options with great success. She was able to create psychic energy dummies to distract her opponent and was a crucial factor in the being's eventual defeat.
4 Blast
Blast is widely known as the S-Class rank 1 hero and the Hero Association’s most powerful asset—as Saitama’s strength is not yet widely recognized. Full details about his abilities and whereabouts were unknown for most of the series. However, when he made his first real appearance during the Monster Association Arc, he displayed great strength worthy of his rank.
His most prominent ability is the power to create portals and traverse freely within them. He can also take things and people along with him or transport each person separately. In combat, he has only really shown his hand against Cosmic Fear Garou, and even then, he could keep up with the Hero Hunter's frightening form.
3 Garou
Garou is a recurring antagonist of the series that has displayed a consistent increase in strength since his introduction. He has been called titles like Human Monster and Hero Hunter due to his admiration for villains and disdain for heroes. His constant evolution saw him achieve staggering levels of power that allowed him to compete with some of the strongest characters in the series.
Early during the Monster Association Arc, he confronted Orochi and was badly beaten by the monster king. However, his fortunes changed after encountering Superalloy Darkshine and defeating the powerful hero. He went on to overpower beings like Platinum Sperm and Sage Centipede, eventually trading blows with Saitama himself in his Cosmic Fear Mode.
2 God
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God is a mysterious being who appears to specific characters in different forms. He seemingly approaches his targets in a different mental landscape each time and offers assistance with the promise of a great amount of power. He is capable of bestowing powers onto others and prefers to keep his existence a secret from those he does not contact directly.
He was responsible for the strength of the Homeless Emperor, the fusion of Psykos and Orochi, and even Cosmic Fear Garou. In Garou’s case, the transformation was incomplete since the latter refused to cooperate fully. However, the Hero Hunter gained unimaginable power nonetheless. Furthermore, he can take away the powers he bestowed onto others, effectively killing them in the process.
1 Saitama
Despite his unimpressive hero ranking, Saitama is undoubtedly the strongest hero in the One Punch Man series. As repeatedly shown throughout the series, he is capable of ending any battle with a single punch if he wishes, and no character in the series has managed to match him so far.
This remained the case during the Monster Association Arc as he breezed through powerful beings like Orochi and Evil Ocean Water, defeating the latter with a single Serious Punch. He went on to face Cosmic Fear Garou, and after the empowered Hero Hunter ripped out Genos’ core, Saitama was noted to be growing even stronger due to a surge of emotion. His encounter with Garou was so destructive that it threatened to destroy the earth, and towards the end of the battle, he let out a sneeze so powerful it cleared a significant portion of Jupiter and exposed its core.