Roller Coaster Tycoon or Mario Kart.

Easy and quick to start up, flexibility in amount of time to invest (1 race/1 roller coaster vs 8 Grand Prixes/an entire Theme Park)

Any 2D Metroid (except the original, that one is just a major pain in the ass)

Boob Wars 2

Oh my God. First now I understood where your username came from.

Yep! Even though Nintendo's my favorite and primary console/game maker, my all time favorite game is a PC exclusive (we don't talk about the Xbox port)

Osu!
Great game to play for short periods of time.

So, are you getting Samurai Warriors Chronicles 3 next month?

OT:  Whatever I feel like at the moment. I don't have games for a specific mood. But chances are it's going to be something either in my backlog or what I'm currently playing.

Bored solo: Start a backlog game.
Bored with friends: Mario Party

This, I just go rob stores or get the police chasing after me or try to steal from the military base.

Any GTA / Red Dead Redemption random roaming does the job.