I regret making this post
My vote would be Myster Dungeon Time/SpaceDarkness/Sky if I would count side games.
For the main series, I would probably go with B2/W2 although it has a lot of remixes of old B/W music. It's original music is fantastic, add to a already good soundtrack.
I also give a special mention to G/S/C, even if HG/SS ruined a lot of the soundtrack.