Fratricide, the special boss battle theme, is a little slower, but just as good overall.
Ascension, the fastest-paced theme in the game, to go in hand with the Boss Rush nature of what you are doing.
The Unholy Parish's theme, Fallen Church, is an entirely different kind of mournful experience.
Breather Boss: Whether you fight Wake or Nevan H'mu at the end of Quagmire, both are very slow and allow for a lot of time to get your hits in before teleporting away. However, that battle is immediately followed by Abyss Itself, and then "Ascension".
, the main boss battle theme.