Cryssie
Ebonclad Enigma
I can't help but be reminded of the Biolizard/Finalhazard here. o.ookay my ideal final boss battle
1: is accompanied by a rockin' soundtrack, but this is a given
2: is long and the boss transforms a lot and stuff
3: is fought in an epic location like on top of the statue of liberty or in an exploding space station hurtling through the atmosphere or something
4: is hard but not too hard (I can beat it on my fifth or sixth try)
5: prior to the battle, your character gets a massive boost in power/goes super saiyan, making it so that during the battle you get to glow and fly around and all
6: is not beaten using any stupid strategy like the bosses in the zelda games are. (ESPECIALLY NOT THE VOLLEYBALL STRATEGY. I hate that.)
Me, I don't really know. Many of the above, probably. Epic in every way, kickass soundtrack, good boss design, fun and challenging (but not stupidly unfair)... and good presentation generally. The game should make you feel that it's a special fight. Atmosphere, y'know? That goes for during the boss fight, before it, and after it.
But I find it tricky to come up with examples of awesome bosses on the spot, and the ones I think of don't necessarily adhere to all the above points...
Portal's boss was fun. The quotes are just beautiful, the method is interesting, and the surrounding circumstances make it mean something.
I'm also a fairly fond of the bosses in Phantasy Star Eps. I & II (which is a good thing, because that game has so few different levels that you inevitably face them a lot, especially the early ones). Fairly nice-looking, great (usually somewhat ominous and sinister) atmosphere, wonderful and memorable soundtracks.
And this is where my brain ceases to work and demands sleep... @_@