Most One Punch Man fans are in agreement that Saitama is undefeatable since he spends the entire series searching for an adequate opponent. With the multitude of monsters and villains fighting against the Heroes Association, only a handful have proved challenging for the great Caped Baldy.
As the title suggests, Saitama generally only needs a single strike to defeat his enemies and, although he is always victorious (so far), some villains have piqued his interest. Fighting alongside the Hero Association, Saitama and his allies go up against Monsters and Ninjas to rid the world of evil. Japanese multi-millionaire Agoni started the Association to avenge his grandson's death, and Saitama joined for the "fun" of it.
TheOne Punch Man heroes are evaluated within four tiers, C-Class, B-Class, A-Class, and S-Class, with the latter being the strongest. Do not be fooled by Saitama's low rank, which resulted from his work ethic and not from his skill level. Monsters, on the other hand, are grouped according to their threat level, starting at Wolf and increasing in severity through the Tiger, Demon, Dragon, and God levels. Ninjas are expected to graduate from the 37th Class of the Ninja Village, with the "Golden" title as the top status.
Spoilers ahead for One Punch Man
Speed-o'-Sound Sonic has made it his mission to defeat Saitama in battle. Regardless of how many failures he has racked up, the Ninja assassin keeps coming back for more. Although not officially ranked, Sonic's ninjutsu skills appear to be within the same level as an S-Class. He lives true to his name, as arguably the fastest fighter around, capable of giving Deep Sea King a run for his money. Additionally, his Bakuretsu Shuriken Moshu-jin technique turns his weapons into homing devices, improving his power potential considerably.
Hellfire Flame and Gale Wind were the top-ranking "Golden" Ninjas to graduate from the 37th Class of the Ninja Village and, since each consuming monster cells, the pair have become formidable opponents. Not only did the monster cells grant them an insane amount of strength, but these assassins possess a unique ability — they are able to transform between their human and monster forms at will. While in his monster form, Hellfire Flame and Gale Wind are ranked as Dragon Level Threats, and Gale can move faster than even Sonic while in this form.
The villain that many One Punch Man fans have a soft spot for is Garou the "Hero Hunter," who was the former prodigy of Silver Bang before being cast out. One of Garou's greatest strengths is his ability to adapt his fighting style, adopting techniques learned by those he has battled in the past. This adaptation is best witnessed when Watchdog Man defeats Garou, who then applied the animalistic style in later battles against the Monster Association. Many fans would classify Garou as an anti-hero because, although he fights alongside monsters, he believes to his core that he is defending them, and he also cannot help but protect any child he meets along the way.
As an executive member of the Monster Association, the power level of this creature is undeniable, and even when Bang, Bomb, and Genos team up, they are defeated. Elder Centipede's strength is intensified by his gargantuan size, which somehow does not impede his speed. His armored coating, or carapace, increases his durability, but even a heat blast from Genos does not hinder this monstrous beast. This Dragon-Class Monster possesses regenerative capabilities along with acidic saliva and was only defeated once Saitama unleashed a Serious Punch.
Monster King Orochi And Psykos
This monster-duo are the leaders of the Monster Association and, together or alone, they are a formidable force. Orochi has the ability to absorb energy from other organisms or directly from the Earth's core, and, according to Gouketsu, is the most powerful being in existence. Psykos shares her colossal energy reserves with Orochi and, in return, her psychokinesis is magnified by the King's special abilities. Giving a new definition to the term "power-couple," the Anime has yet to reveal their full potential strength but the web-manga has painted these two as fierce adversaries.
The first character to survive not one, not two, but multiple hits from Saitama is the Dark Matter Thieves' leader, the extraterrestrial being named Boros. Although defeated in the early stages of Season 1, Boros earned his way to the top of the list for being the first opponent to truly put Saitama's skills to the test. The mighty Elder Centipede was annihilated with less effort, and Saitama had to bring out his Consecutive Normal Punches and finally, his Serious Serious Punch ended things for the Dominator of the Universe. Boros claims that his species retain the highest regenerative capabilities ever recorded, and the ability to project destructive beams of energy. Boros' armor not only provides protection but also dampens his energy - when his full energy potential is released, the toll on the Lord's body is crippling.
Although officially evaluated as a Dragon-Level Threat, most fans regard Boros to be a God-Level Threat, and he will always be remembered as the villain who took three solid hits from Saitama and two special moves to be defeated. Fans may share an affinity for this character because he reflects some Dragon Ball Z vibes into the show, with his Meteoric Burst (Kamehameha) and hairstyles that transform according to his energy levels
Saitama's admirers await the day that his fighting skills are truly tested, as even the battle against Boros was described as "almost a fight." It is uncertain whether Boros really did pose a challenge, as it was revealed at the end of the battle that Saitama was not fighting at his full strength, and the reason why was never revealed. Presumably, Saitama was intrigued when Boros survived the first punch and wanted to see how far he could push the extraterrestrial's boundaries.