Depends on my mood, what I need to "cool down" from, and what I have time for. Sometimes it'll be a fast, action-y game with short "rounds". Other times it'll be some long, drawn-out narrative-driven game.
Still, I can list a few main-stays that I default to in a pinch:
Half-Life - Or, any of the games in the series. It's usually dependent on which scene(s) I wish to revisit.
FTL - As odd as it sounds, I do play this game to relax sometimes. Usually I play vanilla, but at other times I use a program like POKE to kind of "cheat" a little in my play-through.
MechCommander 2 - Sometimes I'll sit down to play one of the other MechWarrior/Battletech games, but this one is the one I always go back to when I need to chill out.
Dota 2 - This I "cool down" with by just playing solo against/with Unfair bots.
Torchlight 2 - This one I do if I have a LOT of time to kill.
Sins of a Solar Empire - Same as with Torchlight 2, but only if I'm in the mood for an RTS.
Terraria - One of the only "Minecraft-like" games I ever enjoyed. I'll usually either do a bunch of back-to-back boss fights or I'll attempt to build some ridiculous structure or contraption.
Sven Co-op - This is the one I've returned to for years. It's my go-to when I want an FPS but don't feel like loading up any of the Half-Life, Doom, or Quake games.
There are others, and they do change over the years, but these are my most common nowadays.
Video games have always been an escape mechanism for me. I've always found comfort in allowing myself to get "lost" in the worlds the game designers envision; far more with some games than with others. This is why, besides music or long walks, playing them has always been a powerful tool in letting off some steam.