I'm gonna rep the underrated FF scores.

Final Fantasy II

This is without question the greatest Normal Battle Theme in the series.
Final Fantasy II - Battle Scene 1 - YouTube

One of the greatest Dungeon Themes in the series
Final Fantasy II OST - Ancient Castle - YouTube

Final Fantasy XII.

The theme of the Great Crystal is absolutely eerie and yet mystical and it's beyond perfect.
Final Fantasy XII - To The Place Of Gods - YouTube

The greatest Final Boss Theme in FF history? Yes.
Final Fantasy XII - Battle For Freedom - YouTube

Final Fantasy X-2

The best ballad in FF history without question.
60-1000 Words (English Orchestral Version)-FFX-2 OST - YouTube

Not one of the top Final Battle Themes but I really like it. I always drag out the fight just to hear more of it and Mr. Taylor's wonderful acting.
Let's Listen: Final Fantasy X-2 - Demise, Shuyin Final Battle (Extended) - YouTube

And to cap it all off...

My favorite FF song is....
Full Final Fantasy IV OSV - YouTube

Yes, the whole album counts. FFIV's soundtrack is a masterpiece the likes of which has never been surpassed in the series.