Most of the field music in Diamond is lackluster. I mean...it's not bad, but compared to the awesomeness that is R/B/Y, they don't hold a candle. And cave music gets on my nerves.
The battle music, on the other hand, is some of the best ever.
Wild battle: Decent, nice tune. A little forgettable compared to the Kanto and Johto themes, but certainly better than Hoenn's.
Trainer battle: Ooh...very nice. I'd rank it up with Kanto's Trainer theme (Johto and Hoenn's were suck).
Gym/E4 Theme: Pretty good. Still not as good as Kanto's though.
Champion battle: NOW we got something. Easily owns any other Champion music. Hoenn's was boring, Johto's was cool, and I don't remember Kanto's.
Trio Battle: While nothing is as good as the Crystal beast music, this comes close.
Cities: I like Canalave simply for being a remix of the Johto end-of-the-game theme. Sunyshore is cool too. Oreburgh is oddly catchy.
Oh, and the Old Chateau is just too creepy not to love.
For field/rural music, I have to give the edge to Upper Coronet (NOT Spear Pillar, although that was cool too). It only plays there, which is a shame because Hoenn's cool mountain theme (Mt Chimney) played in a lot of areas.
~Chibi~