I really love all of the songs used for the B/J moments. There are also awesome ones just spread periodically throughout the show, like Mass Romantic by the New Pornographers and Boy by Book of Love.
But for tonight, I am going to answer with I'm the Main Man by Jason Nevins in the fantasy Emmett&Drew-at-Babylon scene in S4. I love the song and turn it on whenever I am blue or need to work up some energy to clean the kitchen. I also love it in the scene. Emmett feeling so proud. It's just awesome. Tomorrow I might have a different answer about my favorite song though. :)