Fallout 3. If only because the ability to run it would necessitate that we had machines on par with modern PC's at a hardware level and a software market big enough to warrant a port.
That and the fact it was a damn fine game.
? I would select Crytek's Crysis instead of Fallout 3. Fallout 3 can easily run on Xbox 360/PS3 or PC with Geforce 8600 GT GDDR3.