Spore - 16/20 hours

Spore's Space Stage is a genuinely great game. It's a giant virtual sandbox where there's always something to do. You can terraform planets, and that's a whole minigame unto itself where you gather plant and animal specimens from various life-bearing worlds to diversify the ecosystems of your target worlds. Or you can get involved in the spice trade, going from world to world gathering the six different colors of spice and then selling them for ludicrous prices on worlds where the spice is rare. Or you can just explore the galaxy, zipping from star to star looking for rare artifacts and storybook worlds. Whatever you focus on, there's a nigh-inexhaustible amount of content supporting it.

The biggest weakness of Spore's Space Stage is that you can't delegate anything. To make money from the spice trade, you have to physically move your ship to the different production planets. To fight a war, you have to send your flagship out to every single enemy colony. It's fun, but there are no real tools to help you manage the increasing complexity of large empires and a lot of your activities will be limited by the fact that you can't be in two places at once.

Which isn't really a problem, not if you're like me and enjoy the basic experience of playing Spore. It means there's a large gulf between average and optimized empires, but the AI is pretty chill (only a few of the AI archetypes are so aggressive that you can't easily ally with them) so there's not really this big push to squeeze the most out of your colonies.

What I wind up doing is fucking off, exploring worlds and randomly terraforming likely colony candidates until I run out of cash or health or ship energy, then I swing back to my empire and shift a little spice around until I'm flush again, and then go back to whatever I was doing before. It's a huge time waster, but it's oddly satisfying. As you progress, you gain badges for completing various milestones (terraform x worlds, defeat y pirate raids, etc) and those badges unlock related equipment that makes your ship and colonies more powerful. It's a nice way of providing direction in the vast expanse of the Spore galaxy.

I've still got another four hours to go with my current galaxy, and then another 20 in another galaxy with the Galactic Adventures expansion, but I don't think I'm in any danger of slowing down.  The Spore universe is so full of life and activity that even without the traditional space-empire imperatives, just exploring it all is enough to keep me hooked.