So, it seems to be general opinion that the music in SMG is some of the best game music ever, but of that music, which is the best for:
New Tracks (new themes for the game)?
Returning Themes (Airship Theme, Koopa's Road, etc.)?
For new tracks, I really like the music for Gusty Garden Galaxy.
For old tracks, I would probably say either the Overworld 2 theme from SMB3 or Koopa's Road (music for the Bowser levels).