The Title Screen wonderfully shows off the more orchestral flavour of the game's music. Extra points for remixing some of the Super Mario RPG battle theme.
The Boss Battle theme starts fairly joyful as per the series' tradition, but then gets incredibly tense and epic. It combines the almost toonish playfulness of before with some no-nonsense transitions. When you consider this is often how elements blend in the Mario & Luigi games as a whole, it's little wonder many claim this the best boss theme of all.