considering the mods aspect, gotta go with Skyrim and Minecraft for my first two. (do i get multiple versions of the game? if i have to pick just one, i’d have to browse some mod lists to decide for minecraft). and maybe a Dance Dance Revolution arcade machine of some sort.
edit: dwarf fortress is also a strong contender as well
my go to is frequently Puzzles. it is a bunch of classic puzzles rolled into one, and i’ll just go through my favorites on a loop. on a second hand, if you like rogue-likes, Hyperrogue and Shattered Pixel Dungeon are both fun free and offline.
i feel like there are two ways to interpret this question. some games are unpopular because they’re widely known, but considered to be not good, but others are merely niche games that never got widespread recognition.
for the former, Mad Max (2015) got mixed reviews and seems to have been generally regarded as MEDIOCRE, but i still keep coming back to it every so often. for the latter, I wanna plug an old PS2 game called Steambot Chronicles, that has never gotten the recognition it deserves. Its a steampunk mech sandbox rpg, and is legit fantastic, but my partner is the only other person i’ve met that has even heard of it.
rather more of them than on earth, according to the article.