I'm still working on my review for Dark Souls III (there's a lot to process and I want to make sure I get my analysis right), so in the meantime, here are what I consider to be the ten best boss themes across the entire series. Souls games don't tend to have a lot of music, which gives it that much more impact when the music actually does kick in.
10. Ruin Sentinels (Dark Souls II)
9. Maiden Astraea (Demon's Souls)
8. Bell Gargoyles (Dark Souls)
7. Executioner's Chariot (Dark Souls II)
6. Lady Maria of the Astral Clocktower (Bloodborne)
5. Abyss Watchers (Dark Souls III)
4. Ludwig, the Holy Blade (Bloodborne)
3. King Vendrick (Dark Souls II)
2. Gehrman, the First Hunter (Bloodborne)
1. Gwyn, Lord of Cinder (Dark Souls)
I'd argue that this is the most iconic piece of music to come out of a Souls game, evident in the way both sequels' finales return to it like a motif. Most boss themes in Dark Souls are meant to get you riled up, on the edge of your seat, and there'd be no reason to think that the epic final encounter against the antagonist the entire campaign has been hyping up would follow suit. But then we hear this simple piano piece and see Gwyn instead as a tragic figure, one who brought about his own destruction to prolong the Age of Fire. Knowing that your character is likely to follow in his footsteps only makes it more poignant.