What's your favorite boss battle theme?

One of my favorites is the Marx Theme from Kirby Super Star, I would give a link but YouTube doesn't work for me I don't know why.

http://www.youtube.com/watch?v=ycXaFbo5974

Metal Gear Rising. MGRay

i never fought myself to listen to my music

Magus Battle - Chrono trigger http://www.youtube.com/watch?v=U2BYQZvJyyo
I miss the old games with amazing music in the boss battles.

This one

This one probably.

I like Sonic 2's. The evil/sad march. Sets the tone without distracting you from the giant robot that tries to smash you. :D

Sepheroth Fight theme

Not in any particular order:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
(Three were used in a previous puzzle, iirc)

There are many amazing non-battle music pieces for Chinese video games as well. Most Chinese games from the past 15 years or so had good music.
You might also want to check out Heroine Anthem series, 剑侠情缘 series, Prince of Qin, Blade & Sword, Wind Fantasy series, The Legend of Three Kingdoms series, 天地劫 Tian Di Jie series, Heroes of Jin Yong+武林群侠转， Demi-Gods and Semi-Devils， and The Twin Heroes/Juedai Shuangjiao series because they are awesome games.

罗晓音 is also famous for his music for video games.

Devil May Cry 3: Cerberus boss battle

https://www.youtube.com/watch?v=4Kl5-qfi7zM

Donkey Kong's 2 final boss, K. Roll

Xenogears (borderline cheating)

Binding of Issac

Final Fantasy XI

Final Fantasy XI (2)

Final Fantasy XI (3)

Final Fantasy 9

Thats all I can think of right off.

Legend of Heroes: Trails in the Sky - Silver Will (arrange version) It counts since it's used as a boss theme in the third game.

Honorable mentions:
Legend of Heroes: Trails in the Sky SC - Mercliless Savior
Legend of Heroes: Trails in the Sky the 3rd - Genuine Devil
Legend of Heroes: Trails in the Sky the 3rd - Overdosing Heavenly Bliss
Legend of Heroes: A Tear of Vermillion - Octum's Desire
Ao no Kiseki - The Azure Arbitrator
Ys Origin - Termination
Ys Origin - Scars of the Divine Wing
Ys Seven - Vacant Interference
Ys Seven - Legend of the Five Great Dragons

You know what, how about I just say every boss battle theme in a Falcom game, because seriously there are few I don't like and consider superior to pretty much every other game out there.

DMC3

Nothing creepier than the US version of Sonic CD Boss music.

Pokemon RSE Legendary Battle Music
This brings back the memories :D Catching Rayquaza with a Pokeball (0.5% chance haha)

Battle with Onikage

Cheating a bit and using a remix of an awesome one...

https://www.youtube.com/watch?v=ZywnyVRBPG4

Since Cerberus and Vergil's boss themes from Devil May Cry 3 were already mentioned, I'll have to go with this one from Devil May Cry 4... y'know- just to mention it, they're all equally awesome. XD

I think that even after so many years this one wins

If we include final bosses, then It's definitely Mario and Luigi - Bowser's Inside Story.

Love me some Yoko Shimomura.

http://www.youtube.com/watch?v=AB6sOhQan9Y

seems like i don't remember how to make a clicky.

Dancing Mad - Final Fantasy VI

Legend of Zelda: Majora's Mask; Majora's theme, all three stages, but mostly the second.