ZANZA! From Xenoblade Chronicles. Which made the final stage of his bossfight - both the intro cutscene to it and the fight - as well as his death all the more satisfying.
Lavos from Chrono Trigger. I think I may have been a bit underleveled but still. I was sooooo pumped when I finally took him down though.
Oh and whatever boss - can't remember which - that made me just straight up walk away from Demon's Souls. As much as I enjoyed the gameplay, I couldn't take it :P