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.
I like Twin Princes theme a lot better than that, just because it has a stronger melody to it.
I find a lot of boss themes (in any game, not just Dark Souls 3) just kinda gets buried behind the action and gets forgotten after a while, so I like the ones that have strong melody that I can hum myself to. Twin Princes' theme was just immediately catchy, and it's more or less the only boss theme that I still remember clearly.
It's not my favorite boss theme, though. My actual favorite boss theme is down below. Jumpin' Jack Flash from Elite Beat Agents. It doesn't get any catchier than this.