Hmm... Well, taking the scenario literally, there's a few issues:
Firstly, mods - can the game be modded, and if so, how much of it could you do from within the game itself? (Eg. Dev console or the like.)
Secondly, single-player, or multiplayer? - A multiplayer game tends to be incredibly boring without other people logged on, but single-player will get incredibly lonely and tedious given how stupid AI tends to be...
If I were to go with something where I might be able to lead some kind of semi-interesting existence long term, probably something like Second Life...
But as to things that are more typically thought of as actual games... Uh... Hmm. Tough one.
I can't think of many games with enough content not to get insanely bored quite quickly.
( I mean, how many games are still interesting if you've essentially been playing them 24/7 for 2-3 years, let alone longer?)
Something Like X3 might last a while, but the lack of any presence on a human scale would be a little annoying after a while.
Final Fantasy is inarguably great if all you care about is superficial appearances... XD
Um, personally, I guess some kind of RPG, or a star trek game (except that most of the games suck).
So... If I have to choose a game... I'm not really sure.
If I can generally be uploaded to a computer generated environment of some kind, I want one that includes programming tools that I can use from within the environment.
The more things I can re-arrange without having to take the system offline, the better.
But I guess that wasn't really the intent of the question.