If you could own only one game..

[Ares]

Only one game??? I would pick up the better "repleyable" game:

PC: Counter Striker: Source
WII: Probably ProEvolutionSoccer for multiplayer
PS3: GranTurismo5
X360: I don't like Halo, but if i pick up somethink like Oblivion, once I finish it completely, I would never be able to use the X360 again...
DS: Mario Kart