No thread about boss music is complete without Phantasy Star. Or at least the Online portion of it anyway. Never played the single-player JRPGs (sorry, old timers!).
From Seeing the Rough Wave, a track that I feel has yet to be matched in PSO2. That string orchestra!
You Have Nowhere to Go, as aptly named as you're going to get.
The first time you hear
Cry, for Idola the Holy, there is no doubt in your mind that what you are about to face is a primal existential threat.
Jumping to PSO2:
Vanishing Sword, a.k.a. the Quartz Dragon theme nails the atmosphere of fighting a goddamn JET FLYING CRYSTAL DRAGON. The thing is bonkers fast and a real pain to fight for the first (and second, and third, and fourth) time.
la L'inno per il "IDOLA", a.k.a. Dark Falz Luther's theme is quite notable both for its unique soundscape and for how much it takes advantage of the game's music system. They actually have variations of the track for when he speeds up and slows down time, and it's very cool to experience in-game.
Borderless, a.k.a. the Phantom Battleship Yamato theme (yes really) is uniquely one of the only vocal boss themes in the entire series, and it WORKS. I love everything about it. Even the lyrics! Manages to make the fight feel hype as
FUG, despite the actual emergency quest being piss-easy.
Chaotic Dynasty, a.k.a. the Dark Falz Dourumble theme. I don't really know how to describe it. It's just... perfect. Especially for the type of enemy that Double represents (i.e. a chaotic, yet playful, force unbound by logic or reason).
Unfathomable Darkness, a.k.a. the Profound Darkness theme. Besides being a wondrous final boss theme, it happens to also remix tracks from previous games -
Ooze (from Phantasy Star 4) and
"Idola" the Strange Fruits (from the original PSO Episode 3). It really manages to sell that feeling of a real final battle - even if, in the story, all you're doing is pushing back the darkness temporarily.
PSO2 at this point is so chock-full of music that it's kind of astonishing looking back on it now. It's approaching its fifth year of operation and there's no end in sight - along with no sign of any Western release any time soon. A shame, but I can deal. At this point I'm more waiting for PSO3 as I've kind of burnt out on PSO2, but it's nice to see that they're still supporting the game so heavily, even if I'm not 100% on board with their current direction.
I think a sequel (which is inevitable at this point, really) has a ton of potential.