1. TitleWell, this was the track which forced me to buy this game. You could say, 'love at the first sound' or something. The menu screen, and I just stared and listened this eerie and haunting music. No need to say more
2. RemainsDefinitely one of the best tracks. It just fits the atmosphere - ruined and empty city where Naija wanders. A bit melancholic, I love how tempo changes, a bit, like lingering for a moment.
3. Sun TempleHehheh, simply love this! Excellent puzzles in that temple, and this "machinery"-like music playing on background. Unique. It reminds me of "sounds from the past when those machines were build by that old civilization"...
4. ArchaicAquaria theme + excellent music for travelling and exploring. Haunting, like "you never know what you might face after next corner" feeling.
5. Icy WatersThis was the last zone I travelled before continuing to the End. Very beautiful place altogether. Calming music, feeling of 'heart of winter and ice' indeed.
It's very hard to describe music, because if you dont know the "technical" aspects, you have to study how do you
feel towards it... and it seems I could list all tracks here; I listen here as I write and... "this is good too, and this... oh wait a moment, how about this..."

Agh, I surrender.
Altogether, every track is just amazing, and what I especially like, all of them have this "climax", "culmination" or "special part" (I dont know if there's a term to describe this) which makes it unique and exceptional. For example, track "Cathedral" has a very nice beautiful moment at 1:08 - it gave me goose bumbs when I heard it at the first time. "Dark Places", at 0:45. Or "Fallen Breed", such a unique feeling. Or the last battle, heroic theme of the "Worship" (4:37). And the beautiful piano music in the track "Ending"... Plus "Lost to the Waves" with its singing part... damn.
Not to mention this beautiful Aquaria-theme you hear in most of the tracks. I have a saying that "every good game/movie has a own theme music". Aquaria is no exception to this rule.
PS. this is a bit off-topic, but Marian -Travelling -track which came with OST is also extremely interesting. Very nice theme.