I flip flop between Pop and Classical depending on what I feel like hearing at the moment. I'm a huge classical fan in real life and EA has actually selected some very nice examples of this genre for the game, but when I'm really into my game I can play for hours at a time and then I need to switch it up a bit or I'll go crazy. Sometimes I just take my headphones off altogether and play songs from my iPhone.
The other thing that I really like is the song 'Ping and his Checkers' that your sims can learn from the locals in Shang Simla. In my last dynasty several of my sims learned it and they would just randomly start singing it, especially when they were in the shower or bathtub. Very cute!