Surprised no one has said it, but Minecraft. I love it as much now as I did when I was 12. It can be as simple or as complex as you want it, especially if you start playing with mods. Then there's servers such as Hypixel which for all intents and purposes are an entirely separate game in and of themselves.
I'm sure part of it is nostalgia, some of my fondest gaming memories were playing Minecraft with friends, but I still find it to be an excellent way to relax.
(Tangentially related, anyone else remember when waiting 3 months for 1.2.5 > 1.3 was an absurdly long update time for the game? Different times.)