Uh, this is a tough decision. Majority of my gaming time has been spent on 4 specific gaming franchises: Sim City, Civilization, Pokemon, and The Sims.
I'd probably have to go with Sim City 4 Deluxe. I'm just a huge nerd when it comes to building cities in it and I can use the same terrain to come up with different layouts every time I decide I want to wipe a city and rebuild.
Granted, ask me again in 4 months and I might answer with a different one of these 4 games. My desire to play these games is fairly cyclical.