I like completing challenges. I love being able to max and super-max skills, and collecting. Making the 'perfect' sim. And then, once I've done that, making the 'perfect' sim in the least amount of time. And then, once I've done that, turning to the challenge board.
Of course, building is my favourite. I don't often play the houses I build. I play-test them, but that is about it. Although, I do have a really nice world that is nearly done. I think I'll definitely play that for a while.