Pandemonium Warden? I never played FFXI but I heard the story of how a group fought it for 18 hours straight and failed to kill it. Also there was another one I can't remember, some angelic thing that was practically unkillable because it pulled a Yiazmat-Renew whenever it felt like it.
Given how FC's have the likes of T13 on farm, how would you feel about a "true" superboss in the game?