I know a lot of people love phase 1 better, but I feel that phase 2 is just as great because it really highlights the chaos going on at Lothric castle.
Ornstein is my favorite boss in all of Dark Souls for no specific reason, and the fact the Dragonslayer Armour kinda resembles him, makes the Dragonslayer Armour a very close second.
Yeah I like the first phase music too but this one is also greater in its own right, it does its job really well in instilling panic into the player with how the butterflies are now getting in on the battle aswell and how the armor is also going 100% too