I only really play two at any given time. I almost always have one primary Beat-That-One game. Used to be that was all I needed, but recently games have been getting longer and longer (Fallouts, Elder Scrolls, etc.) and sometimes the grinding in the middle or a long lull in story just causes me to need a breather.

Insert game #2, the never ending/drool on yourself/vegetative state autopilot game. This rotates around, but usually centers on one game for a year or more at a time. Past highlights have been Flower, Skate 2, and most recently Fallout 3. I can play any one of these games without ever turning my brain on. I used to play Skate 2 and spend an hour and a half on one handrail trying to get a trick string right.