When we drove across the country, we only had one CD handy, which was my MLP FiM remixes. I think we listened to it about 60 times...
My fiance now has a favorite pony song, which is this one:
http://youtu.be/NWd5x09gWoEBut he always makes it a point to say "20% cooler?"
He's seen one of the episodes with me, where RD and Fluttershy have the Find a Pet Song. He finds that catchy too.