The isekai subgenre of anime transports the viewer and main character to another world where anything is possible, and while "isekai" isn't an official anime genre, the concept is widespread and cohesive enough to count as an informal genre of its own. Such series often act as a form of escapism, especially where power levels are concerned.
Some isekai series make their protagonist practically helpless so that they must survive with nothing but their wits and ingenuity, such as Subaru Natsuki of Re:Zero fame or Myne from Ascendance of a Bookworm. Other isekai series go in the opposite direction, thrilling viewers with an openly overpowered hero. However, one OP isekai hero in particular stands above the rest.
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What Is Isekai Anime?
The informal genre of isekai involves relocating the hero to a brand-new world, often a fantasy realm or even a computer game world or the far future, where they have little to no chance of returning home. These heroes must adapt to whatever this new world throws at them, making new allies and fighting powerful enemies. While some isekai heroes survive by the skin of their teeth, others become far more powerful than they were in their previous lives, dominating everyone in their new role. This may be a form of escapism for viewers who resent their tedious life at school or work, where they feel invisible and unappreciated, and they become the hero in a world where the old rules don't apply.
Honorable Mentions for Most OP: Ainz Ooal, Azusa Aizawa and Toya Mochizuki
A handful of OP isekai heroes have won over the hearts of viewers, with Ainz Ooal Gown of Overlord fame being one of them. He's a video game-based isekai hero who becomes the lord of his own domain, and all his followers, most of all Albedo the succubus, are fanatically loyal, regularly patrolling his fortress. He aims to slowly remake the world in his favor, and speaks casually of influencing or overthrowing entire nations and religious organizations.
Azusa Aizawa the benevolent witch is another runner-up -- an overworked office employee who aims to enjoy her new, carefree isekai world for all time. She spent three centuries grinding her XP to the maximum level and can now invent and cast powerful spells with ease. She also has incredible physical strength and stamina.
Toya Mochizuki is yet another OP isekai hero, a teenage boy who has the favor of God himself and a smartphone in hand while exploring his new isekai realm. He's not ambitious at all; he just wants to explore the world and make friends, so he uses bloodless spells to keep his enemies at bay and keep the peace.
The Most OP Hero: That Time I Got Reincarnated as a Slime's Rimuru Tempest
Rimuru Tempest is the lead of That Time I Got Reincarnated as a Slime, and he is exponentially more powerful than any slime -- or indeed, most isekai characters of any type. He started off with only a handful of abilities granted by Great Sage but gained even more over time, some of them passive, some active. He can resist heat and cold, use telepathy, wield offensive magic of all sorts, regenerate his flesh and most of all, absorb another being with his Predator skill. This allows him to analyze the creature and gain its attributes, and even morph into its form. When Shizu allowed it, Rimuru used Predator on her, granting him the power of Ifrit and Shizu's likeness and boosting his own charisma.
However, Rimuru's greatest strength isn't just his enormous combat power. He is a balanced isekai hero, being a diplomat and politician as well, and he is rapidly learning how to form and run an entire diverse nation of monsters who live together in harmony. Rimuru single-handedly created the entire Jura Tempest Federation, and he built it up not by slaughtering his enemies, but by having the grace and wisdom to recruit them, even ravenous orcs or vengeful ogres.
Rimuru also continued to build international ties with human and dwarf kingdoms, and his political power is now reshaping the entire world around him, gaining the attention of everyone from the Holy Western Church to the Demon Lord council. Rimuru has transformed entire species and nations with his influence -- and he didn't need divine intervention or cheat codes to do it. It was all through his own ingenuity and good heart, and that proves that in spirit, he is truly overpowered.
Rimuru has few weaknesses aside from his shrinking ignorance of existing political situations and world leaders, and his rapid rise to power makes other parties, such as Clayman, feel threatened. Rimuru's dominance is a double-edged sword, as he learned that he can't make friends with absolutely everyone. He must accept this and proceed with no loss of enthusiasm, but so far, he's proven he is more than up to the task.