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.