Dark Souls 3 - Abyss Watchers.
Not only is it a beautiful piece of music on its own, the way it fits the fight, and caps off the adventure you've been on up to this point, is sublime. While we never get told much about the bosses in Dark Souls, it's clear the Abyss Watchers were once a force of good, and the soft melancholic rise of the score as we watch them locked in eternal battle with themselves gives the impression things are not as they should be, and as phase two begins, the music swells up, doubling down on this feeling.
It's perfect.