for normal bosses id say FF6. FF7 and 8 boss tunes are good but the FF6 one (espicially the black mages version.). for final bosses id say dark messenger, the necron fight and maybe im a lion. the extreme is good. ive not completed FF6 yet but i have heard dancing mad and its a great tune. i like otherworld. its amost impossible for me to choose a favourite. FF music is bettet than any video game music ive ever heard and there are so many good tunes. my top 5 would be (in no perticular order.)
the necron tune.
dark messenger.
maybe im a lion (im almost certain that name was a tribute to deep purples song "maybe im a leo." the lyrics go "maybe im a leo but im not a lion".)
FF6 boss battle.
and the neo X-death tune or the bizzaro sephiroth one.