I've probably heard Clair De Lune more times than any other song.
At first I was going to say Pink Floyd - Dark Side of the Moon, the entire album. Then I started thinking about how many times I played "Don't Worry Be Happy" on cassette tape when I was 8 years old, and then I started thinking about how many times I listened to the "Oliver and Company" soundtrack when I was ~6.
Then I started thinking about the number of hours I spent playing Final Fantasy - so perhaps really I've heard more Final Fantasy music than any other. Or was it Diablo II, afterall, I played that game for something like 2 years.
No, throughout my life I've probably heard Clair De Lune more times than any other song. I've always had relatives that played it. So I heard it from childhood. Heard it in movies, heard it in concert, played it on the computer, heard my wife play it, and eventually, I played it myself - practicing over and over.
I'm going with that, but it's a tough question. I spent an awful lot of time in the Diablo II cow level.