I've never seen anyone mention it, so I'll say the ending for Kyokai no Kanata (Beyond the Boundary). It's so good, and the way they lead into it at the end of the episode is great. Unfortunately you don't see that part in the linked video.

https://animethemes.moe/anime/kyoukai_no_kanata/ED-NCBD1080

Recently, the ending for How Raeliana Ended Up in the Duke's Mansion. Again, the way they lead into it every episode is great. And the one episode where they changed the song got me super emotional unexpectedly.

https://animethemes.moe/anime/kanojo_ga_koushaku_tei_ni_itta_riyuu/ED1

Alternate song (pretty big spoilers in this one): https://animethemes.moe/anime/kanojo_ga_koushaku_tei_ni_itta_riyuu/ED2

Honorable mention: All of the Eureka Seven OP/EDs. I wouldn't quite say they're underrated, but since the show is almost 20 years old now I'm guessing a lot of people haven't seen them.

https://animethemes.moe/anime/eureka_seven