Most difficult by FAR was Krosis. However it was also the coolest moment in skyrim for me. Im on my way to do the stormcloak quest when a dragon i didnt notice landed on my horse. It was my first elder dragon and it gave me some trouble. After i finally manage to beat him I go towards the dragonwall and BOOM omfg its krosis and he just nearly raped me in one hit. How did i kill him? Some wandering prophet of bathalac or what ever (who also tended to one shot me) comes up and starts attacking krosis. God dude so much went down on the way to that stormcloak mission.
Morokei is the hardest and the easist was the guy in Rivendele or something like that. He came out of his coffin and I owned him and not until I looted his carcass did I find a mask to know he was one of those dragon guys.