when I took delivery of my Model S, the first song was Imagine Dragons "I'm on top of the world". I did not specifically make that choice, slacker did for me, but I thought it was fitting.
When I took deliver of my wife's Model S, I am pretty sure the first song was some disney movie song, as I had both of my little girls were with me at the time, and they never let me listen to anything else when they are in the car.