It's one of these hands down:

Cowboy Bebop
Ghost in the Shell: Stand Alone Complex
MADLAX
RahXephon
Samurai Champloo
Read or Die (OVA or TV, doesn't matter)

If you notice the music constantly in a series (which you usually don't in these) it stays in the background and fits the scene it's in perfectly. You might notice it all the second or third time through, but you might miss a lot of the music the first pass. More importantly, if there is enough of it and it fit properly, when you hear just the song, you can imagine the scene it was in perfectly. That's why these are it, no question. If you wanted to get really specific its either Bebop or Ghost in the Shell, both of Yoko Kanno's work, so it doesn't really matter.