What about the fight against (is it omega or ultima) weapon in FFX
I just found the challenge invigorating, and watching my auron with his ultimate weapon and damage break doing his overdrive tear him to shreads was great.

Other than those i suppose all the jenova battles (simply for the music) and any Final Boss Battle is an easy bet for a fantastic boss. (some of the optional bosses were gd, i.e Ruby weapon, yojimbo etc)