Without a doubt, Draugr Death Overlord. Morokei was hard but these guys are unbearably so. Use of shouts, EXTREMELY high damage, Lots of HP/damage reduction. Words of advice - If you haven't faced one of these yet and you aren't at least lvl 30, RUN, run like hell.
This. I haven't had too much issue with anything yet. x3 Ice bears at once was a close call however. And the Death Overlord, during the first two swings of my Legendary Skyforge Steel Greatsword was going to be just another battle. But he hit me with a shout that disarmed me. Sent my sword flying. So now, here I am, 2 handed warrior nord with a bow backup weapon and a fire magic spell that might, just might, generate enough heat to light my cigar.
Spent the next 15 minutes running around the room blasting him with magic trying to kill him and frantically looking for my sword.
Killed him with magic and the majority of my potions and food in the long run and found my sword hidden away behind a pile of rubble. It was a tough battle but taught me a lesson. Now I keep a backup one handed blade and shield just in case I come across something like that again