Brotherhood is the show I recommend when someone asks for “the one”. It starts as an adventure about two brothers fixing a terrible mistake, and it keeps widening until it’s about war, guilt and what people owe each other.
What makes it work is control. Sixty-four episodes is long, but almost nothing is wasted, and small details from early on matter later. The villains have real reasons, the side characters get their own arcs, and the action always serves the story.
If you only ever watch one long anime, this is a safe choice.

