Perfection. If not that it's as close as I have seen.
Animation is revolutionary. It's combination of 3d and 2d is seemless, like oil paint character art came to life. Beyond being amazingly kinetic and gorgeous it achieved a subtlety that you'd be hard pressed to even find in live action.
The story is great and was delivered at just the right pace. No need of any prior knowledge necessary and everything organically feeds into itself. Everything is done with an amount of intention and attention to detail such that the world it builds truly feels connected naturally and smoothly. Because...
All the characters feel genuinely human. Laughs/jokes are never forced and neither are the times it gets dark. It knows how to sit with emotions so nothing ever feels cheap. Dialogue feels human.