tx12001
I will not tolerate failure...
the most powerful NPC in terms of Level and stats is Miraak followd by Arngeir who are both level 150, In terms of who is harder to beat its a toss up between the ebony warrior who is level 80 and Kartsagg who is level 90. but really if you are gnna talk about all chaarcters encountered in the game it is probably hermaeus mora or sheogorath or even your own character if you level up far enough.