Was just flicking through my ff ost's and a couple of those memorable battle theme's hit me. Whats your fave one, the one that made you feel you had to continue just to see the demise of the final fiend.
To jog your memory lets takes a look at some.
FFVI - Dancing Mad. i personally loved this one perhaps it is my fave I dunno but it has sorta a gothic ring to it but at the same time it makes you feel like this is the end and you have to beat this.
FFVII - Sephiroth Orchestra I forget its real name but this was also a classic one for me, the way it just goes "SEPHIROTH!"
FFVIII - Hmm I actually cant remember ff8's sorry maybe you can :o
FFIX - Forgot its name but its also a good one, once it gets going that is and the tempo rises.
FFX - Otherworld. Not technically the last boss theme, but it's there when you fight Braska's final Aeon. I really got into this track it fuelled Tidus hatred for Jecht imo.
So in conclusion Dancing Mad would have to be my fave.