If you could take one game to a desert island...

[SS-18 ICBM]

If there's an Internet connection, TeamFortress 2.

 

[Kan' Sharuminar]

If there's multiplayer - any online game as it allows me to call for help.

Although, I admit, the general antipathy of folk online means it's very unlikely I'll ever be considered seriously.

 

[civplayah]

The Sims 3. It's so simple, it never gets old. All you have to do is keep your people alive, but you can do so much more.

 

[Maniacal]

Such as stranding them on a desert island and watching them duke it out for survival as they starve to death in their own filth.