If I'm just relaxing, probably Arma 3. It's editor is like playing with toys for adults. I love making missions or armies that aren't balanced or made for other people. I do this little mini-game with myself where I build up an army starting with only pistols and a few guys. Then as you beat more competent opponents, like maybe it starts put with poorly equipped pirates and looters and goes up to fighting the United States or PLA, you get more and more equipment and fight on progressivly larger maps. It's very fun, and feels like being a child again. Plus, maybe it has a bit of Metal Gear Peacewalker inspiration.