It's not unique, but it's my own - Civ6 Civilization Theme Tier List
This is of course subjective to my personal preferences. Needless to say, almost all of these are masterpieces of world music adaptations in their own right, but on the flip side that does mean the field is much more competitive than usual in game scores. I fell in love with Civ6 music almost a decade ago, and despite barely playing the game anymore, I often still use the civ themes when working out or driving, they are just that good!
The final ratings are by averaging out where I'd rate them in each age (relative to other civs from the same era since rating Ancient vs. Atomic on the same scale is futile) For example, Arabia starts off somewhat slow and uninteresting (to me) in ancient (probably C or D) but quickly makes up for it in medieval age and onward with some truly great arrangements. Because as mentioned above there is so much competition, good songs will end up in B or C tier just by virtue of not being AS good as the rest. Being "mid" to "good" in this particular tier list is especially punishing, so those of you who find one of your favorites there hopefully can forgive me for it.
I rate these by a few loose criteria, in addition to just how much I like each one personally and how they can make me patriotic about places I've never been:
How well they "evolve" through the ages. Themes that feel alive and that they match the age they're in well while still remaining true to their underlying cultural music gain points. Themes that remain more uniform across the ages lose points (sorry, Babylon).
How well they can crescendo to epic riffs and then decresendo back down if needed, makes the song more dynamic and gains more points.
Gain points for incorporating unique instruments and/or iconic music (yes Scotland is one of my favorites).