Valeran or the 3 dragons in Bappasalar Cave. You are more unprepaid for that battle, specially for the Bappasalar Cave, there is no easy way to recall and redo that again.
I don't know if we should count being unprepared as a factor. Since if you have to fight the Undermother or Lord of the Pit unprepared that fight gets a dozen times tougher too.
Well, looking solely at the stats, Stridsibum should be the hardest enemy in the entire game.
Thanks for pointing that out. I didn't even realise he was also a possible enemy in the game and the one with the highest level (not counting the Horror if his level grows too big) He looks very tough and now I wish I realised earlier that I could have fought him since I can't do it anymore after completing the quest. It makes sence though that the being that created goblins is one of the strongest beings in the game. He certainly deserves a spot on the list.