My least favorite is the ancient/classical era. Medieval/Ren/Industrial is great. I got mixed feelings about the modern era. I get lymonds description of "cacophonous noise", but I also feel that's kinda what you want near the end of the game when everything (should be, but rarely are) is about to be decided. The modern music creates a sense of urgency that I like.
The problem is that instead of being a dramatic ending, the modern era tends to be a laggish hell where the game is already decided with lots of End-Turn clicking and no interesting decisions left. And then the modern music is just annoying just like all the sound effects in the modern era that you have to suffer through to reach the victory screen. Graphics(read: ugly roads!) looks terrible too.
Which game falls off harder, Half-Life or Civ?