For me, it's the Arch Electromancer in that ruined castle before you get to Sanguine, in his daedric quest.
Hardest.Enemy.I.Ever.Fought.In.Skyrim.
Not Harkon, not Miraak, not Alduin, firstborn of Akatosh, the Winged Shadow and Devourer of Worlds.
It was that blasted bandit Electromancer in a...