One Punch Man broke onto the scene many years ago with an enthralling first season. At first glance, the anime seemed nothing more than a gag, but with every episode, it became clear that there was more to it than what met the eye.
One thing that became very clear in the early episodes was that Saitama is ridiculously powerful, and he could beat any villain rather easily. So, to make things tough for him, the creators have to introduce villains, who are armed with outrageous abilities. However, not every villain in One Punch Man has the same level of power.
8 The Deep Sea King
The Deep Sea King is the main antagonist of the Sea Monster arc. He emerged from the sea after his underlings failed to complete the task that they had been given. The Deep Sea King was extremely sadistic, and he would go to any lengths to hurt people.
As a monster, he was much more powerful than an ordinary human. When he was hydrated enough, he could transform into a bulkier version that granted him extra strength. He was able to take on the likes of Genos and Puri Puri Prisoner, without any problems. Eventually, he was taken down by Saitama.
7 Overgrown Rover
The Overgrown Rover has the appearance of a massive dog, but it is not nearly as friendly. It has tons of special abilities that make him one of the stronger monsters in the Monster Association.
The Overgrown Rover also has high speed and durability. In addition to these abilities, it can also shoot energy projectiles that can completely vaporize Demon-level threats. It was able to overwhelm Garou physically, although the latter had not completely become a monster.
6 Sage Centipede and Evil Ocean Water
The Sage Centipede and Evil Ocean Water were sent by God to destroy the heroes. The Sage Centipede was a ginormous centipede, and it dwarfed the Elder Centipede, which was no pushover either. Despite its size, the Sage Centipede was able to move quickly and surround its enemy in no time.
The Evil Ocean Water fought alongside the Sage Centipede. It was considered to be one of the most powerful members of the Monsters Association, and after absorbing large quantities of water, its size increased by several folds. It is fair to say that these monsters would have wreaked havoc if Saitama and Garou hadn't killed them.
5 Homeless Emperor
Unlike most members of the Monster Association, Homeless Emperor has the body of an ordinary human being. This was a major weakness, but the powers bestowed upon him by God made him a fearsome long-ranged fighter. Homeless Emperor could create energy spheres that could cause a great deal of destruction.
These energy spheres could be created in all sizes, and he never grew tired even after using them multiple times. He met his end when God took away his power and life force.
4 Black Sperm
Black Sperm made his first appearance in the Monsters Association arc. He is a creature, who belongs to an unknown race. Black Sperm relies on his ability to combine his cells to take down opponents that a single cell cannot beat.
His two most notable forms are Golden Sperm and Platinum Sperm. Platinum Sperm is made up of trillions of cells, and in this form, he can defeat even the strongest of the S-rank heroes barring a few. Like other powerful monsters, Black Sperm has great physical strength, regeneration, and speed.
3 Psykos-Orochi
While Psykos and Orochi are two different characters, they combined to form an incredibly powerful being, which is why they're included as one entry. On his own, Orochi was a dragon-level threat, and he was considered to be the strongest monster on the planet.
​​​​​​Psykos was a potent esper, but she wasn't as skilled as Tatsumaki. So, it wasn't difficult for the S-rank hero to deal with her. Once Orochi combined with her, the two formed a monster that was exponentially stronger than their individual powers. One of their attacks managed to slice the Earth's surface, which shows the disparity between them and the rest of the monsters. In order to beat them, several S-rank heroes had to work together.
2 Boros
Boros was the main antagonist of the Aliens Conquerors arc. After destroying numerous planets with his strength, Boros and his Dark Matter Thieves made their way to Earth.
In all of his previous travels, Boros had never found an opponent who could face him in a battle, but that changed when he met Saitama. Boros possessed various abilities such as high regeneration, the ability to create shockwaves, and incredible agility. He was able to withstand several punches from Saitama, and the latter had to use a Serious Punch to stop him.
1 Garou
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Garou is among the most popular One Punch Man characters. He had a very idealistic view of the world and how problems should be solved. Before becoming the "Hero Hunter," Garou used to be a student of Silver Fang. Due to his talent, it was extremely easy for him to learn all the techniques and become stronger.
With every fight, Garou is able to evolve and become more dangerous. After going through monsterfication, Garou's physical attributes increased enormously, but he didn't hit his peak until God granted him new abilities, which made him arguably the strongest villain in the series to date. His power increased so much that if his clash with Saitama had not been relocated, the entire planet would have been destroyed.