at high levels, priests. I've had one cast blizzard then slaughter me with wall of flames. they have seemingly endless magicka and access to basically doomsday spells. Giants swing sticks at you. Granted, they are big sticks, Master level destruction spells beat it. I mean, (as far as I'm aware) the toughest boss, miraak, is a priest. He trumps alduin, simply because you can do nothing on even legendary besides clear skies shout and win thanks to the nord heros.