By New Sonic, I assume you're talking about Adventure onwards, correct?
Then I'll go out on a limb and say it: The Adventure games, as well as Sonic Heroes, had some of the best music the series has to offer. SA1 actually has my single favorite soundtrack from the Sonic series, which is no small feat. It was a logical evolution of the classic style, but it rocked harder and was just cheesy enough to really capture the feel of the levels. It was also fittingly nostalgic.
Case in point: SA1 offered both the kickass
"Bad Taste Aquarium" (for Hot Shelter) and the sentimental
"At Dawn" (for Speed Highway). These pieces are quintessential Sonic, and fit right alongside classics like Sonic the Hedgehog 2's soundtrack. Unfortunately, since then, there have been several duds in the soundtrack department (see Sonic and the Secret Rings, the Sonic Rush games), and I'm not sure about too many of the post-Heroes Sonic games as a whole, but at least initially, the music wasn't half-bad.
I really hope they stick to the J-Pop/ J-Rock style that the original games had in future entries, because Senoue really did a good job when Sonic debuted in 3D.
I also agree that Sonic Unleashed's model is the best 3D one, but I still prefer the classic model from the old games, so it's not really a step up overall.