Spore. Games don't come any more ambitious than Spore and although the premise of the now in-famously delayed game and magnum opus of Will Wright is fairly simple, the actual realisation of that concept has proven incredibly difficult.
The idea behind Spore is this; you are God, the Alpha, Omega and Almighty. You are omniscient, omnipresent and capable of creating a rock so big you can't possibly lift it. Then you can lift it. You're God and that type of feat is your bread and butter.