Gist of list is: quality maps that never made it to rank for one reason or another. Some of the maps under hard don't have many favs simply because of the raw skill required to play those maps.
Another reason for low favourites one some is simply empty tags, making actually finding the songs hard.
Solid mapsDJ Sharpnel - Marunouchi survivor by ampzz
https://osu.ppy.sh/b/294015* The galvenize's difficulty is underrated. Very solid older style map. its also harder than SR shows
** 7 favs
Demetori - U.N. Owen wa Kanojo nanoka? by Louis Cyphre
https://osu.ppy.sh/b/844718* Unsure if going for rank or not. Similar to 2011/2012 "metal" touhou maps in style
** 70 favs
DJ'TEKINA - Internet bitch P*Light Remix by dannarz
https://osu.ppy.sh/b/502183* The unranked (and subjectively speaking, better) map of the song
** 62 favs
Foreground Eclipse - Flames Within These Black Feathers by Saten
https://osu.ppy.sh/b/147625* Has full spread but never got ranked despite being high quality
** 95 favs
Krewella - Say Goodbye
https://osu.ppy.sh/b/959406** 54 favs
ChillGirls generation - oh! by cryo[iceeicee]
https://osu.ppy.sh/b/282379* Was popular in lower ranked lobbies a good while
** 76 favs
MemesBiBi x DragonForce - Through the Cutie and Panther (Metal Mix) by Birkis
https://osu.ppy.sh/b/826947* Dragonforce meets bibi. Mapping mimics either arles or hollow wings style. Pretty hard map
** 8 favs
Manuel - Gas Gas Gas by LefafeI
https://osu.ppy.sh/b/969764* Initial D. Although new map, its in older style
** 23 favs
Joe C. - Kyle's Mom's a Big Fat B**ch by Naitoshi
https://osu.ppy.sh/b/539793* Yes. It's that song from south park and map is actually pretty good and fitting for being a meme song
** 4 favs
Reading challengesKobaryo - Pumpin' Junkies (Kobaryo's FTN-Remix) by EvilElvis
https://osu.ppy.sh/b/773398&m=0* Very fast hard to read map
** 22 favs
Caravan palace - lone digger by Sheepcraft
https://osu.ppy.sh/b/824970* 124 bpm, but plays like 248. similar to ranked caravans in how it plays
** 100 favs
Kishida Kyoudan & The Akeboshi Rockets - Jiyuu e no Sanka by Nyquill
https://osu.ppy.sh/b/213135* Alternate map, very very smooth
** 73 favs
gmtn. (witch's slave) - furioso melodia by Urushi38
https://osu.ppy.sh/b/432032* Easier version of furioso by unranked(unsubmitted) mapper urushi
** 56 favs
FujuniseikouyuuP - Invasion of stray cats by val0108
https://osu.ppy.sh/b/588523* One of last 0108 before break. Solid reading style map
** 166 favs
HardCamellia - werewolf howls. by Necroh
https://osu.ppy.sh/b/743464* Featuring difficulties by Arles and Musty. Brutal map
** 11 favs
C-Show - U.N.Owen wa kanojo nanoka? (TO-HOlic mix)
https://osu.ppy.sh/b/705199* Collab with necroh and arles
** 14 favs
Helblinde - Grief & Malice by Necroh
https://osu.ppy.sh/b/588599* Another quality gravemap
** 119 favs
GigaP feat. Reol - Gigantic O.T.N -Big Death Edition- by eXceed
https://osu.ppy.sh/b/662767* Solid map, not seen in multis sadly
** 15 favs
Camellia ft. Nanahira - Bikkuri Sasetara Okoru yo! by Yogurtt
https://osu.ppy.sh/b/677922* High(ish) bpm challenge map
** 18 favs
Camellia feat. Nanahira - EDM Jumpers by Thexes
https://osu.ppy.sh/b/748880* Solid challenge map
** 5 favs