I was looking to create a similar post myself today but then I discover this thread. Since there are too much awesome music we should make top 10 lists though...
Favorite themes
Peace
Honorable Mention: Arabia (6th next to Byzantines and Romans but really should be mentioned)
5. Byzantines (Tie with Rome. Beautiful chorus.)
5. Rome (Tie with Byzantines. Feels classy enough to make me feel like a Roman in the Empire's finest years hoping for eternal and infinite flourishment.)
4. Songhai (very underrated as mentioned above. Very well designed civ when it comes to aesthetics.)
3. Ethiopia (I just like everything from Ethiopia in this game.)
2. Egypt (another underrated one.)
1. Mongols (captures the magic of the Eastern world, the nomadic and rough lifestyle of the Mongol people, the tragedy and legacy of Genghis Khan, the brave expansion of the Mongol Empire.)
War
5. Japan (took me ages to start liking it. It has a great Samurai feel into it.)
4. Sweden (Tie with Ottomans. "Overpowered" is the best word to describe it...

)
4. Ottomans (Arguably better than the peace one which is already top-tier. Tie with Sweden.)
3. Brazil (Best of BNW soundtrack. Evil samba? Pedrobear? Bring it on!)
2. Spain (Knightly and active.)
1. Ethiopia (3000 years of Ethiopian history and last independant African native state stare into you, European invader...)
LEAST Favorite themes
Peace
5. Venice (Well there's a European feel into it, and Enrico Dandolo's old age, could be done better though...)
4. Poland (Badly remixed Christmas carol.)
3. Korea (Sounds like Korean instruments randomly put together without really trying hard.)
2. Aztecs (Boring.)
1. Inca (Sounds like a song for children.)
War
5. Morocco (Poorly done civ when it comes to aesthetics.)
4. India (Gandhi knows his peace-loving armies perform the worst war anthems.)
3. Germany (Awesome peace theme, but this one is bland. I was expecting a fierce war music along the lines of the Prussian military tradition and Nazi supremacy.)
2. Korea (I don't hear Arirang anywhere and practically a worse version of Korean instrument mixture.)
1. Songhai (Awesome peace theme, but not creepy at all and just plain poor.)