Currently for me it's Shadowbringers (Final Fantasy XIV: Shadowbringers).

https://www.youtube.com/watch?v=UqKTq-0nIr4

Here's the thing that makes it so great (spoilers for XIV, obviously):

Everyone originally thought this song was about the heroes - the Scions of the Seventh Dawn - because we knew Shadowbringers was about going to another reality called The First and becoming the Warriors of Darkness. So the heroes were hoping to be "riding home", "one brings shadow, one brings the light", etc.

In reality, the song that defines the expansion - is actually about it's villain and it is their theme, the antithesis of the base game's Answers which is the theme for Hydaelyn and her heroes.