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

EDs:

• Fire Force S1 ED1
• Shadows House S1 ED1
• Komi Can't Communicate S2 ED1

OPs:

• The Vampire Dies in No Time S1 OP1
• Land of the Lustrous S1 OP1
• Blue Lock S1 OP2
• Case Study of Vanitas S1 OP1

Not really sure what qualifies as underrated/overrated,.. BUT in terms of things I can find on YouTube and general sense of amount of discussion I've seen,.. I think the OPs get more attention than the EDs of these series when I've tended to like the EDs better

Seisenshi Dunbine's ED because power ballads are fun

Gundam IBO S1's ED feels James Bondy with the swung feel and the strings (incidentally Aria the Origination's OP Spirale, also great, kinda feels James Bondy to me but more for the chromaticism,.. ANYWAYS)

and yeah, this was a stacked season (with competition like Bocchi the Rock) but I think my favorite ED of that season was Do It Yourself's ED because of its ultimate comf (kinda gives me the same feel as Nichijou's first ED)

Paranoia Agent's OP and ED are creepy and really good. Probably not underrated but I don't know anyone else IRL who's seen it.

I also really like Haibane Renmei's ED.

One I never see mentioned anywhere is the ED for Asterisk War. Mediocre show but you get to hear Maaya Sakamoto sing in English.

I love the opening song of Ergo Proxy. And the opening sequence, by the time, was on par with the new Trigun's one.

I don't believe I've seen much discussion of the ED for Tonari no Seki Kun (https://youtu.be/oIKB7i6fcmk), and it is amazing.

Mobile Suit Zeta Gundam - Mizu no Hoshi e Ai wo Komete

Space Dandy - Viva Namida

SSSS.DYNAZENON - Imperfect

Underrated or good? Because I have no idea how people on large perceive or rate ops/eds.

Serial experiments Lain ED is one of the most underrated ones. Even its lyrics hit deep. Kinda addicted to this song for a few weeks now.

• Hitomebore - Band harassment (Chihayafuru s3 ED)
• Ghost in the shell OST - Kenji Kawai

The ending of Asobi Asobase. Actually, scratch that, the whole anime is underrated as hell!

The ending of Konosuba S1, both OPs and EDs of Kumodesu, specially the EDs. And that Naruto ending retelling Obito's life too.

Just off the top of my head.

Oh, and I linked the anime version of all those songs, but you REALLY have to hear all of Kumodesu's ED1 for the full experience.

In/Spectre Season 2's OP, Yotogibanashi, doesn't get nearly enough attention, much like the show itself.

I really like Ancient Magus' Bride OP1. Junna just belts that song and when it comes back at the end of the first half of the season, it fits so well

Oh I know one. The ED from Stella no Mahou. Stella no mahou is a fairly mediocre show that you may or may not decide to watch at all, and even if you did, you probably forgot all about it once the season ends. The ED is great though, it's electronic house(might be wrong) music, rather than a standard anisong. If you're after some EDM anisong, it won't disappoint.

I'd like more people to talk about many aspects mushishi since I think it should be considered an all-time classic. Sore Feet Song is one of the few that stuck with me for nearly a decade now.

Canaan's OP Mind as Judgement is really really good and I rarely hear people talking about it, much less about the anime itself.

The OP of Subete ga F ni naru.

Kimino Shiranai Monogatari from Bakemonogatari.

Kawaki no Ameku from Domestic no Kanojo

I haven't seen much recent anime, but as a connoisseur of funk I give my accolades to:

• Rosario Vampire s2: https://www.youtube.com/watch?v=FHojVhh6k40
• Space Dandy: https://www.youtube.com/watch?v=IZzSxLdLVp4
• Burst Angel: https://www.youtube.com/watch?v=5PAbRpzoEX8

Because it amuses me to throw in something really obscure: "Genesis—to the Traveler who Wanders in 'Zero'", the OP from the first part of Genesis Survivor Gaiarth, an OAV that no one remembers.

Ao-chan Can't Study. Weirdly really good OP for a short comedy ecchi anime.

the Ed for unbreakable machine doll