Minecraft. If I had to pick a song specifically, I'm gonna say Subwoofer Lullaby. It just really captures the calm, familiar feeling of "living" in Minecraft. Farming, renovating, adventuring, spelunking, etc. Whenever I'm working on something, ranging from just building the first rendition of a house in a new world to developing an automatic redstone-powered bamboo farm to supply a mass smelting furnace at the bottom of the world that uses water elevators for access, the soundtrack is unmatched.
It also just helps that it's an iconic game for people my age and I've spent years on and off playing it since alpha.