I can't answer that,because I can't pick just one.I have too many that I like.Also if I heard that one song everyday,eventually it wouldn't be a favorite of mine anymore.
It would probably be like Wii Shopping music, Pokemon Eterna Forest, (nonspecific) Zelda music, or...How Bad Can I Be? (Cantonese version).
I've shared HBCIB a lot so I will provide you with this beautiful song, for now xD
Someone beat me to Teenagers so I guess The Ghost Of You by My Chemical Romance. It's already basically on loop all the time.
Or Helena.
Or just Three Cheers For Sweet Revenge in general, I love that album basically more than I love myself.