Actually, I'm quite fond of the entire soundtrack as a whole. I have almost the entire set.
But if I had to pick, I'd have to say it's a tie between Call Me, and Chicken Bone. Both are very good songs and I'm quite fond of the originality of them both. Wo Quo Ni Coin would have to be in second. I like the transition from Japanese to pseudo-French. It's friggin' great, I tellz ya! ^_^
But I think you can't really enjoy just one song from that soundtrack without enjoying the entire OST as a whole.