The best song in this list is J-E-N-O-V-A. I was listening to the songs through, in order, to try and make my decision, but it only took two seconds of that song for me to realise there was no contest, really. It's simply one of the best battle songs the Final Fantasy series has ever had. The key, I think, is that it alternates between minor "verses" and a major "chorus", mixing despair and hope, as fast and intense as you like. The game's Main Theme is another song that exploits the major/minor contrast to good effect.
So... I'm not voting for it. It already has 14 votes; easily enough to go through to the next round. My vote goes to my next favourite, Cait Sith's Theme. Even though I don't really like him as a character (as mentioned recently elsewhere), the song just fits him perfectly, and it's a great composition that is always getting stuck in my head.
Special mentions to Rufus' Welcoming Ceremony and It's Difficult to Stand on Both Feet, Isn't It?