Fallout New Vegas, I suppose. Or a later installment, assuming I won't die in the next few years.
I'm assuming the GECK is included, in which case I'd spend all my time learning how to mod the game effectively. Since the afterlife is for eternity, I'd have a functioning scale model of the entire earth after a few millenia...
If you had asked me the same question two years ago I would have looked at you like you were mad when you mentioned 'no multiplayer'. I'm a social person, being stuck in an awesome game world with no one to share it with is my own special kind of hell. Fallout 3 was the only game that ever changed that for me. Maybe it's the world, maybe it's the characters, maybe it's just the idea of a post apocalyptic society... it seems alarmingly real.