tough one. I actually don't think I have a fallback game anymore. I can be burnt out on ANY game, even ones that I love. Then again, I always try to never "over-play" them, to avoid this. My fallback game USED to be Oblivion, but even that I find I have a hard time booting up.
Really, even games with fun MP like TF2, halo 3, or either MW can burn me out if I play them too much. I guess I just can't understand that grindy mentality of sinking hundreds of hours into a game to level up, or whatever. How can you not get sick of it?