I was thinking of this topic long time ago.
Sometimes b-sides are real bangers and even better than the title track. Share here your favorite ones ! So we can discover underrated / unknown / forgotten tracks !
I have many, the list can be long but I start with some of my tops :
Wish Aryu would come back
Even in 2021-22, BUCK-TICK can still manage to pump out amazing B-side Ballads like this… KOI is just an absolute tear-jerking soul experience… 
Great thread idea!
I’ll probably revisit this later, but for now here are some bangin’ tunes that come to mind:
Even though they only released this one CD, the B-side of aim’s single “LOST VOICE” is an absolute killer - no wonder the musicians later re-recorded it in their next band GALEYD.
BUCK-TICK is notoriously great at B-sides, so it’s hard to take a pick. Today I’m in the mood for the haunting “Asylum Garden”.
Last but not least, leave it to LUNA SEA to casually drop a B-side that actually became a signature liveshow staple, and also the name of their fanclub.
残-ZAN-“D.P.Y. Mix” is my favorite Gauze-era Dir en grey remix and one of my favorite Dir en grey B-sides in general. That’s saying something coming from me. アクロの丘 (K.N.Y. Mix) is okay, just not my thing. ゆらめき “S.N.Y. Mix” is just absolutely dreadful.
While we’re on Dir en grey, 腐海 (Fukai) off the かすみ (Kasumi) single is a straight banger. Even though the band has been using the version with new lyrics for years, I still prefer this one.
To round out the set, the second re-recording of Ash off the 脈 (Myaku) single is my favorite of them all. Note that the order goes Behind the Mask vol.2 1997 -SOL’FINSTERRE- → 「楓」～if trans･･･～-> 脈 → 人間を被る.
Bonus: Cant’ forget Unknown･･･Despair･･･a Lost
OMG, YEEEESSS!!! ASYLUM GARDEN IS SO GOOD!!! 
Here are some of my favorites from girugamesh. I didn’t include any songs from full albums.
Their b-sides from 【空虚の器】～虚策型円盤～ 通常版
These from their first mini【獄-初犯型円盤-】
Dirty Blue Blood from 【理性壊乱】-乱調型円盤
These from Reason of Crying
Vision fromイノチノキ
takt from INCOMPLETE
Go ahead from gravitation
And finally these from 鵺 -chimera-
The B-side on this single is way better than the main track haha.
Here’s my underrated b-sides ballad selection !
I only listened to them somewhere back in 2009,
but this song caught me directly. Beautiful relaxing song.
These traditionals instruments ! One of Alice Nine best songs. Pretty relaxing too.
This one is not so unknown but I could’nt not to put it here.
Well, I didn’t put rules for mini albums since we can’t really talk about b-sides but I accept it for minis and ep. And this one, god… Res favorite song and among my favorites in vk/jrock ever.
Oh crap I forgot to add the b-sides from Crying Rain really loved these.
One b side that comes to my mind is this one by Nega from their Vanitas album
Great taste.
OMG!!! THIS SONG!!! YEEEEEESSSSSS~!!! this was at 1 point my GO-TO song for whenever I was needing a song to vent out any sadness I was feeling at the time! 
This song… Just… this song. I can’t. 
blue sea with a breezy lonely island beach
Such a beautiful song.
There’s Senjo ni Hanataba Wo by Alice Nine…
Futomei Ningen by Dezert
Goddess by The GazettE
and my favorite girugamesh song: