Hard to choose hard choose the sonic Games always get the music right. it just has that natural flow to it. But i would have to say either SA1,or Sonic unleashed.
After watching and listening to some of the Soundtrack, I say that Sonic Adventure has better music then Adventure 2, to revise my choice somewhat. Colors music is still awesome.
Adventure 1 and 2 had some of my favourite tracks from the sonic series but as a whole their soundtracks weren't anything too special.
Also, 06 only had two tracks I would actually want to listen to more than once making the rest of the game's soundtrack very forgettable. At least it ends up going well with the rest of the game then.