Simple enough question. What is the most difficult boss you've faced in a game?
I have two that I found to be ridiculously difficult.
First up is Dark Bahamut from Final Fantasy X. That fight was ridiculous. You spend most of the battle trying desperately to keep your party members alive and free of status effects and there is very rarely any good opportunity to attack. Still I managed to beat him after a hell of a long fight. Definitely the hardest Dark Aeon I found.
Second would be Sephiroth in Kingdom Hearts. He attacks relentlessly from the outset and all of his attacks are very damaging and hard to dodge. I actually don't think I managed to ever beat him.