>>49945052 (OP)
I don't think anything is underrated in 6-8 so let me jump straight to PoFV and continue from there.
PoFV: The Mound Where the Flowers Reflect (Pre-Battle Conversation theme)
Genuinely doesn't even sound like ZUN-composed music at times. This is a song that doesn't really need remixes as it's amazing enough as it is, though granted it is a relatively simple melody taken the title theme, but the composition is magic and unlike anything ZUN has implemented into gameplay sections of touhou.
MoF: Sealed Gods (Title screen theme)
Oddly nostalgic, song perfectly encapsulates MoF atmosphere and the bit with the rain and rain droplet sounds elevates it further.
SWR: Flawless as Clothing of the Celestials (Tenshi pre-battle theme)
Perhaps the most majestic song in the touhou franchise.
SA: The Earth Spirits' Homecoming (Ending theme)
Genuinely one of the best songs on the soundtrack, a wonderful symphony of melodies and instruments that create the perfect ending theme for the game.
UFO: Sky Ruin (Stage 3 Theme)
Despite all the great stage themes in this game, this is the one stage theme that has probably increased my love for it over the years. It's a perfect song for exploring a giant flying treasure ship, and I think it genuinely is another case of Chinese Tea where the stage theme is better than the boss theme.
UNL: The Eternal Steam Engine (Staff roll theme)
So happy, uplifting, cute and energetic. Love the irish folk sounds!
DS: Bell of AvΔ«ci ~ Infinite Nightmare (Photo Theme 4)
It's wonderfully melancholic and bittersweet, very understated too for a song called Infinite Nightmare.
GFW: Great Fairy Wars ~ Fairy Wars (Three Fairies of Light's theme)
I don't know if this is underrated and overlooked, but I found the composition of this song to be so interesting and complex. It has a bit of everything thrown in and it oddly works so well!
TD: The Hall of Dreams' Great Mausoleum (Stage 5 theme)
Used to dislike this theme because of how mellow it was, but nowadays it's so incredibly elegant and atmospheric to me. I feel like more and more these type of stage themes starts to appear post-TD and some might not like it, but I certainly do.
HM: Futatsuiwa from Gensokyo (Mamizou's theme)
I think this arrangement of Mamizou's theme hits harder than the version from TD.
DDC: Illusionary Joururi (Benben & Yatsuhashi theme)
Now THIS is how you do a proper music poltergeist theme RIGHT cough cough Phantom Ensemble cough cough.
ISC: Raise the Signal Fire of Cheating (Title/Menu Theme)
It's a good menu theme sure, but I especially love that one bubbly electronic bit that comes on later in the song. I just like it when ZUN experiments a little with different instruments, this is one of those moments!
LoLK: Unforgettable, the Nostalgic Greenery (Stage 1 theme)
I don't think ZUN has ever gone this hard on a stage 1 theme. Goddamn is it intense, and I love the pounding sounds especially.
HSiFS: The Magic Straw-Hat Jizo (Narumi's theme)
There are several underrated songs from HSiFS. Just candy to my ears, I love the melodies and I love how kind of rough and powerful it is, especially for a character like Narumi. Unpopular opinion, but I love the electric guitars here, especially their contrast to the ZUNpets.
WBaWC: Tortoise Dragon ~ Fortune and Misfortune (Yachie's theme)
This entire soundtrack is underappreciated and overlooked, and I could've chosen several tracks. But Yachie's theme is seriously slept on for being as good as it is, and I'd go as far as to say it's better than Idolatrize World. I love how creepy, elegant and majestic it is, and I love how electronic it is as someone who looooves electronic music. The choir is just the cherry on top. Genuinely one of the best songs in the entire franchise.
UM: Ore from the Age of the Gods (Misumaru's theme)
Another banger modern touhou stage 4 boss theme that gets overlooked because... whatever reason. The song has genuine final boss theme vibes and arguably she is the most difficult boss in the game even in stage 4, so it kind of fits.
UDoALG: The Path to Yomi Where None Turn Back (Hisami's theme)
Hated this from first listen, ended up being my favorite from the whole soundtrack. When you really listen and take apart all the instruments and melodies, it is such a fascinating, complex song. ZUN's description for the song says " A song where the strange tempo stubbornly pushes you from behind " and I 100% understand that!