It changes too much to really have a definitive ONE song.
Right now, and probably the winner all time for me is the first song that I ever heard of his: AFL
I've been listening to it a lot more lately. It's the perfect song to go into BOOH2 after FCOL (I usually play them in order in a playlist).
It's the perfect song. A wicked intro, awesome lyrics, awesome keyboard work, awesome BR vocals, awesome guitar work... I could go on and on.
I'm gonna press submit now before I change my mind.