I can't really decide on a particular 1 through X order, so take these as occupying the same slot.
Openwaters2? (First area after Home Waters, I love singing the lil' tune near the end of the song where it begins to start its loop again... With Naija of course.)
Light (First section, great way to start off Aquaria.

)
Worship5 (C'mon, 'tis epic. )
TheBody (Spiffy song for a spiffy area.)
Mithalas (The city section anyway. Really seems grandeur and somber at the same time.)
Lost to the waves (Is nice, a tad bit short but it gets the job done.)
'Course, all the music is masterfully done. But those are the ones that stick out the most in my mind.