I don''t think it's underrated at all - probably the whole opposite - but, literally, everytime I tell someone to watch it, they never heard of it beforehand, so I'm throwing in Made in Abyss. It has amazing worldbuilding and every episode kept me hooked.
Also, the Digimon movies, best from Adventure to Frontier - even though I've heard that Frontier isn't liked a lot, I think it has it's charm - and Last Evolution: Kizuna!
And one of my personal guilty pleasures, when I really need positive vibes: Miira no Kaikata. Very cute and fun! :3