Chewed on this one for a bit, good question. Most of the music in the series fits the setting and is composed well enough.
That said. I've noticed I'm rarely fond of the diner themes (Ajichi and Yamaji Soba from SI; Bar Man Mo, Dou Jiang Diner, etc. from S2). I also have to be in a certain mood to appreciate Y.A.D.A, hah.
The only songs I really dislike (can't decide which is worst) are: White Dynasty (Night), Beverly Hills Wharf (Day-- Xbox), and the rich married couple song from S3.
Edit: I'd never given Ohshu Soba a chance. It really picks up, and I strike it from the list! Reminds me of Pigeon Cafe: unexpected drum beat surprise.