Vol. 3 - some of the jazz / blues stuff from Mystery of the Blues is probably my most-played music from these CDs, whereas my favorite tracks on the other albums are score cues (then again, the other albums have far less source music on them).
I love Volume 3 but I was bummed by the omission of The Man I Love. That song was pretty darn important in the episode -- the game-winner! --so I was really surprised not to see it on the soundtrack album.