Okay so let's say for the sake of argument that you had to pick just one game, or maybe one from a few different genres if it's that difficult to pick an undisputed favourite.
What would it be for everyone? An all time classic (Ocarina of Time/Final Fantasy 7/Mario 3/Halo)? A cult game that you totally loved despite it's many faults? Or something in between that you just spent hours and hours wasting your childhood years playing?