Flip-Swap Galaxy. A simple, tension-filled remix of the SMB Underground Theme that knows its place as background music and lets you focus on the action.
On the other hand, there's the Beat Block Galaxy theme. While it's also a remix of the Underground theme, rather than staying in the background to let you focus on the action, this tune is the action: it appears in levels that change (such as gravity changing) according to the beat of the music. This has the effect of making the tune a massive Ear Worm.
The Gobblegut battle theme is epic and tense, perfect for a fight with a giant space dragon. In contrast, the Megahammer battle music is surprisingly Ear Wormy, as is the boss theme for the fights against Digga-Leg and the Whomp King.
And then there's the Squizzard battle theme, which is a tense desert-themed tune, but what's really notable is that it has a unique mix of the Fire Flower theme!
The final star, referred to as The Perfect Run, might be insanely difficult, but getting to hear Gusty Garden Galaxy, Good Egg Galaxy, Bowser's Galaxy Reactor, and the observatory music from the first game once more makes it much more bearable!