It 100% depends on my mood. If I want to relax, explore, grow a character, then I'll definitely go with an RPG or MMO like Requiem Bloodymare (very bloody MMO :D look it up, just came out) Oblivion, Morrowind, or Diablo 2
If I'm in a tactical mood and my friends are available I hope on BF2 (Large scale FPS) or something of the sort. There aren't enough shooters on the PC out there like BF2 that are populated.
If I'm angry, or stressed, or anxious, I need to hope on an FPS. I prefer F.E.A.R. (Perseus Mandate or Combat) because of the feel when you kill someone; CS is great and all, but there's nothing like the blood decals, lighting, tasteful gibbing (not the crap like Quake), and AMAZING particle effects from bullets hitting drywall or grenades picking up so much dust mid-combat that you can't see.