Well, it used to be halo 2. But, unfortunately it is no more. I played a fair bit of halo 3, and it was good, but it wasn't quite the same. I had a lot of problems with halo:reach, and don't even get me started on halo 4. So, the last time my xbox live subscription was up for renewal, I just let it go. Basically, leaving me without a go-to-game.
Generally, I'll play though a game, then just move onto the next one. Now and then, I'll go back to something from my childhood, like spyro, or crash bandicoot, but I rarely replay a more recent game. It would have to have been something I really enjoyed.