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.

Dynasty Warriors, They are my "Dumb fun" game. I usually just turn off my brain and "grind" even though I usually call out every game for grinding or unnecessary combat.

But something about that series makes me pick up every one and waste tens of hours just killing the same-ish soldiers over and over.

Shepherd's Crossing 2, for the DS. I've put a truly ridiculous number of hours into it over the years, and keep coming back.

It's not popular in the farm sims because a key part is butchering your meat animals so you can feed your dogs.