I don't understand how you guys do it. I mean, yeah sure, Ultimecia wasn't that tough compared to other end-game bosses, but if your Level 100, that would still be tough. Cause it's been proven that the lower level you are, the EASIER it is to actually take down monsters, because they grow at such an exponential rate compared to your party members.
Yeah, and the main thing are junctions to strength, magic, and HP. Seriously. Maybe a few ELM and STA junctions depending on the boss fight. Lionheart, My Final Heaven, and Wishing Star are really all that you need to kick ass. Omega Weapon was only tough because he has an instant-kill move. Yes, it affects your whole party and you cannot block it (Unless you have a holy war, *wink wink*).
Onto the main topic. I'm assuming we get to choose non-plotline bosses as well, so my vote will be Nemesis from Final Fantasy X. Never heard of him? You need to complete all the Arena capturing quests in order to unlock him.
But damnit, he was tough.