Wild ARMs 3 Crossfire Sequence. Not the one on the OST. This one is also Janus' theme, but it's one of, possibly the only track, that's in game but not on the four disc OST. I know it's simple, but simple is good too.
https://www.youtube.com/watch?v=M40NhO9ORhE
*edit*
Here are some of my other favorites.
Final Fantasy 4's Dreadful Fight https://www.youtube.com/watch?v=bp4-fXuTwb8
Final Fantasy 6's Decisive Battle https://www.youtube.com/watch?v=Ozx5p3tbxAg
Final Fantasy 6's Dancing Mad https://www.youtube.com/watch?v=JbXVNKtmWnc
Final Fantasy 8's Force Your Way https://www.youtube.com/watch?v=54r8t_5ayto
Final Fantasy 8's The Extreme https://www.youtube.com/watch?v=RVkcmx2l3WA
Atelier Totori's Go Go Totori https://www.youtube.com/watch?v=Q1yEQX3_-OE
SMT Nocturne;s Dante Battle https://www.youtube.com/watch?v=SMX09FSyHqs
