This one is great. Makes me wanna complete that Xenosaga playthrough I started a long time ago.
Speaking of battle themes (I'm presuming this includes boss battles), there are couple that I found amazing when I first heard them, and I still like them to this day:
Digimon World 3, Boss Battle: http://www.youtube.com/watch?v=Drbe1T2pe7k
Don't know if this is because I was a huge Digimon fan as a child, but I love this game and I remember just leaving the controller when this music played so I can listen to it.
Final Fantasy 8, Force Your Way:
I first played FF8 somewhere around the time I started learning to play the bass, and I was amazed by the bassline here. Still my favorite FF boss music to this day.
Chrono Trigger, Battle:
Damn I was so surprised when I heard this while fighting Gato, especially considering that prior in the game you hear entirely different types of music. Also, I'm a huge sucker for basslines.
Also, don't know if this counts, but Crash Team Racing - battle mode music is awesome:
(rest of the battle music is a variation of this theme)