"Battle shonen," or boys action series with a heavy dose of fantasy ("shonen" is Japanese for "boy"), is a staple of anime/manga and the mediums' most famous genre. "One Piece," "Naruto," and "Demon Slayer" are all quite different series at first glance, but they're also all battle shonen. And the very best of this genre? Hiromu Arakawa's "Fullmetal Alchemist" (adapted into the anime "Fullmetal Alchemist: Brotherhood").
"Fullmetal Alchemist: Brotherhood" is set during the early 20th century in the fictional (but European-themed) country of Amestris. In this world, the science of alchemy has applications in the military, medicine, philosophy, etc. But as with real science, there are limits: Alchemy relies on equivalent exchange, so you can only create by sacrificing something of equal value.
As such, when alchemist brothers Edward Elric and Alphonse Elric, having apparently never read "Frankenstein," try to revive their dead mother Trisha, they are punished for committing the sin of "human transmutation," with Edward losing his right arm and left leg, while Al's soul is bound to a suit of armor. Hence, to heal themselves, the brothers vow to find the universal elixir, otherwise known as the Philosopher's Stone.
The action of "Fullmetal Alchemist: Brotherhood" is spectacular; every fight is cleverly staged and beautifully drawn. Compared to many other shonen series, "Fullmetal Alchemist" stands out for its propulsive pacing and thematic depth. The central question of the series, derived from the Elrics' original sin, is if you can measure the value of human lives as you would other materials. Alchemists take the power of creation into their hands when they transmute, but they are held down by thermodynamic laws. If you could bypass those restrictions, then could you truly become God? The show's villains think so.
Now, "Fullmetal Alchemist: Brotherhood" is actually the second anime based on Arakawa's manga, but don't feel pressured to put it off. "Brotherhood" is more widely available, a more faithful adaptation, and most would agree it's better. (Not that the 2003 "Fullmetal Alchemist" anime is bad, to be clear.)
At the end of the series, Edward (really Arakawa) declares that "there's no such thing as a painless lesson." But while the Elrics' journey may be full of pain, "Fullmetal Alchemist: Brotherhood" will be a trek you'll want to take again and again.
"Fullmetal Alchemist: Brotherhood" is streaming on Netflix, Crunchyroll, and Hulu.