to be honest, i dont really want to identify a single, go-to, ultra favorite song. most of the time i'd have a song or a set of songs that i'd need to hear daily and eventually other songs will replace my current favorites. i particularly love songs that i get from tv shows and movies because i associate them with the scenes i find most memorable.
right now, i'm loving a lot of songs from the Skins soundtrack, especially Hummer by Foals (okay, i heard it in the Unseen Secret Party episode, and it quite blew my mind). i mean, just watch the epic-ness at the scene they used it on!
some other favorite songs i've had:
500 Miles by The Proclaimers (heard in recurring episodes of HIMYM)
Sound of Silence by Simon & Garfunkel (heard in Watchmen)
Dragonfly by M. Craft (heard in Skins season 1)
Come Undone by Robbie Williams (heard on MTV 6 years ago)
1901 by Phoenix (heard in New York, I Love You trailer)
Marla by The Dust Brothers (heard in Fight Club soundtrack)