On the flipside of the thread of the most disappointing bosses, there are also some amazingly epic bosses.
Name your favourites!
For me, Lei Shen is a pretty epic boss. The cliche' is you weaken the boss, and he teleports into his contraption to power himself back up, but Lei Shen powers his pylons up with himself.
Not necessarily a "boss" per se, but the final level of Fire Emblem 7 is pretty epic, fighting all the morphs with the epic weapons, and finally killing Nergal with assassin Matthew with Silencer.