The isekai anime genre is known for two details: reincarnations and overpowered protagonists. With some of these series, however, the concept of ridiculously powerful main characters is taken to an entirely new level. Thus, their premise centers around the character essentially waltzing around in a world where nothing really challenges them anymore.
Some of these shows have heroes that are instantly overpowered, while others are developed into these power levels along their adventures. Their abilities also have differing effects on their supporting casts and their worlds. Here's a list of some of the best and "strongest" isekai anime and where they can be streamed.
Overlord Is the Ultimate OP Isekai
The dark fantasy anime Overlord is considered one of the best isekai ever, even among those who dislike the genre. Reborn as a grossly overpowered skeleton knight, former gamer Momonga is feared throughout the land -- and for good reason. With nothing stopping him, including morality, Momonga discovers that he can do whatever he wants.
Overlord succeeds by being an almost deconstruction of the isekai genre, showing how dark things would become if someone was reincarnated into a world where they were suddenly given tons of power. As the king of isekai, it can be streamed on Tubi TV, Hulu and through both Funimation and Crunchyroll.
In Another World With My Smartphone Features an OP Cellphone
In Another World With My Smartphone sounds like an outright parody of isekai, which just shows how ridiculous the genre has become. Protagonist Touya is reincarnated from his world into a fantasy dimension, retaining only his smartphone. To power the device, he's given a grandiose amount of magical energy to power it, easily making him OP in his new world.
What works for the series is that it's fairly chill and easygoing, logical given its hero not really being in danger of anything. This makes for a relaxing show that makes up with atmosphere what it lacks in thrills. In Another World With My Smartphone is streaming through Funimation, Crunchyroll and VRV.
Reincarnated As a Slime Has a Strangely OP Hero
Starting a whole new subgenre of isekai was That Time I Got Reincarnated As a Slime, which is as crazy as it sounds. A normal salaryman is reincarnated into a lowly slime named Rimuru, but this blob has a dangerous secret. With a special technique at his disposal, he can copy the attacks and appearance of anything he consumes.
Reincarnated As a Slime has been praised not only for making its weird premise work, but also for its worldbuilding and characters. The former is especially given a level of detail typically unseen in isekai anime. The series can be found on Funimation and Crunchyroll.
Wise Man's Grandchild's Hero Is and Isn't OP
Wise Man's Grandchild has a hero who's more than adept in magic, though he certainly falters in other areas. Shin Wolford was once a Japanese salaryman before being reborn in another world as a baby. He's adopted as Merlin Wolford's grandson, who teaches him ridiculously powerful magical techniques -- but not common sense.
The show succeeds by waxing and waning between the protagonist being practically overpowered or not, as well as adding in the age-old "magical school" element that's popular in fantasy anime. Wise Man's Grandchild can be found on Funimation, Hulu and Crunchyroll.
Cautious Hero Is Also an OP Hero
Cautious Hero blatantly states that its main character is overpowered, but his caution curtails some of this. Summoned by the goddess Ristarte to defeat an evil demon lord, Seiya is a hero with power stats that far outrank anyone else. The problem is that he's incredibly cautious about everything, making using his immense power an issue. It works by transitioning from pure comedy to eventually becoming a more serious and epic affair. This is especially true as the story reaches its end. Cautious Hero can be streamed through Hulu, Funimation and Crunchyroll.
The Rising of the Shield Hero Features the Rising of an OP Hero
The titular protagonist of The Rising of the Shield Hero isn't initially overpowered, but he definitely becomes that way. Betrayed by a seeming friend, Naofumi the Shield Hero builds a cadre of outcast allies. Driven by pure hate, he goes on to gain not only power, but also the trust of the society that had rejected him. It's become controversial over the years, but Shield Hero is still an interesting and unique take on isekai, especially given that its hero being overpowered is a development rather than instantaneous. The anime can be found on Funimation and Crunchyroll.