This discussion may contain spoilers!
Season 1 brought us a number of really impressive background tunes in a variety of styles. Hard as it is to choose, what's your single favorite piece of background music and from which episode is it. Only background music to locations you can actually play in count: no special music that only plays in certain cutscenes and none of the end credits or musical intermezzos. We should have a separate thread on those (maybe I'll even start it myself at some point), but I'm trying not to have them automatically eclipse the rest here. To jolt your memory, locations of each episode included (edit: d'oh, I left out the Imposing Door and Driving 2.0 locations!).
I'm pretty torn myself. Both oval offices tunes, the background music inside the war room, pretty much all of the Reality 2.0 music and the Moonscape tune (which I somehow only started to fully appreciate after listening to the MP3) all come very close. But ultimately my personal favorite is the Toy Factory music. I can't find the words to explain why, I just know this is the tune I keep going back to listening to again and again and again.