Do I still keep my achievements? Took me a long time to get all those. If so, then I would keep my current Dead Rising 2 playthrough save, I do not feel like starting at level 1 again and I'm really starting to enjoy the game now that I'm like level 22 and know what I'm doing.
Assuming you often connect to X-box live, if anything ever happened to your gamertag you could very easily recover it by selecting the "recover gamertag" option on the dashboard when you are not signed into a profile, then put in the windows live id and password associated with your gamertag and Voilà!
Anyway onto the question at hand:
This has already happened to me twice. The first time I accidentally deleted all my saved game data from my N64 memory card. The second time happened when three of my "friends" stole most of my Ps2 games, my controllers, and my memory cards.
If I had to choose one game save to salvage it would probably be my primary Mass Effect save.