I think Galaxy's pretty much the hands-down winner (it gets my vote too). Let's leave second favorite soundtracks open for discussion as well, because mine would be Super Paper Mario. It has lots of great music that fits the level themes perfectly, and it's the first time I've heard actual recurring themes for particular characters, bar Grodus (that was in a much more minor way anyway). The different remixes of Count Bleck's theme were great.
About Galaxy itself, I like...
-Good Egg (very fitting and epic for a "mission" air)
-The ghost theme (nice and creepy, would also fit something else like a post-apocalyptic ice area)
-Gusty Garden (what everyone else said; very generous and epic. I'm not sure what I mean by generous, but that word came to mind)
-Melty Molten (intense, firey, and catchy)
These are just my particular favorites; I love almost all of the tracks, including some like the ice/fire theme, Dusty Dune, and Purple Coins.
Don't forget Toy Time Galaxy, Gateway Galaxy, Space Junk Galaxy, That remix of the SMB3 athletic theme,
Bowser Stage, Metal Bowser and a bunch more I forget, oh and, the best is Super Mario Galaxy (no contest)