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I would say Ascendance of a Bookworm is my favorite but it’s not The Best world.
Disc World would be the best. Naussica from the valley of wind, and Made in Abyss had very good worlds but they lack depth.
Read the mangas, both of them are absolutely packed with a ton of detail that never made it into the animes.
Naussica especially, there’s a whole worldwide ancient apocalypse buried in there on top of a VERY unique and well thought out ecosystem.