get President Kennedy to wear a helmet, it's a very specialized time machine. It will work exactly once, and bring you back to the development stage before a game has launched. You can't change the team or the studio, but you can have complete conceptual control over the game from the earliest testing until it finally goes live.
With that in mind, what game would you pick? Would you go back to a game that's been canceled to see if better development early on could save it? Would you try and tweak something in your favorite game that always altered you? Would you try and make an older game more compatible with future design? Or would you go for a game that you never cared for and try to make it better?