Warframe is my most played game on Steam. I don't think anything will ever surpass the time I have in it.
(Yes the game has many forms of grinding, which adds to the total amount of hours spent.)

But I love this game, it's come such a long way since it was released. I think my favorite thing to do is show new players the ropes and getting to help them succeed as well as watch their reactions to all the quests I did years ago. (Some I may be watching again because I legit forgot what happened lol.)

It's free, if you are worried about inventory slots, just buy the cheapest Platinum purchase and trade Prime parts to other players to get more Platinum. You do NOT need to spend a bunch of money on this game, the main grinds have to be done manually, so Platinum is mostly for cosmetics and inventory slots.