I'm not talking about a large faction, like Imperials or Vampires. Who was your favorite enemy that you've fought in Skyrim?

Me, personally? Harkon, an honorable mention being the Ebony Warrior.

Boss fights in Legendary Mode are always fun for me. Harkon is the only vampire in the game that actually uses his transformation or his powers. Additionally, as a Paladin, I employed my Sunlight Spells to the maximum. It's always great to cut lose and unleash all of that fury! Every skeleton slain with the Dawnbreaker would blow up in holy light, dealing more damage to Harkon. I would power bash him with Auriel's Shield, slice at him with my sword, dual cast a Vampire's Bane in his face, and finally end the fight with a Sunhallowed Arrow, right through the heart, of course!

I enjoyed fighting Karstaag. It was the most difficult and most enjoyable battle I've ever had.

Nazeem

Hmm. I'm off to fight him right now, actually. I've beaten the Ebony Warrior on Legendary, it will be interesting to see how that compares.

I find fighting Karstaag easier than Ebony Warrior because I just hope to one of cave ledge where he and his minion can't reach me and incinerate him.

Therefore I choose Ebony Warrior as most favorite enemy, not just because fight difficulty but also the fun, his mysterious background (Redguard who know Tu'um and want to go to Sovngarde).

Ebony warrior.

Kaarstag.

Harkon was fun too depending on my character (some characters had a harder time because they didn't have Dawnbreaker, spells or the bow).

Tsun.

Harkon. Well, to be fair, that's the only challenging enemy I have ever fought. Mercer? Karliah and Brynjolf got him. Isran? Stabbed him in his sleep.

Vyrthur. Let's face it, he literally brings the house down!

I brought Dovahbear and Vyrthur died from fall damage.

From the little I played so far, I have to say Malkoran. He just hits so hard with the destruction spells, and then he comes back for a second round. Plus he's got minions. It was a pretty good feeling when I killed him.

The emperor, he's just so accepting of death when you encounter him. It's a sudden surprise in how he is all calm and actually polite towards the bloody assassin that comes to kill him. Actually wanting to talk first, then turning his back to look out the window enjoying one last dawn while you move in for the kill.

That's how you accept your fate, and one reason I killed the original contractor. Out of respect for a man who accepted his fate.

The Gauldur trio but only if you're sub level 30. Especially the teleporting one. He will shout you away, kill your followers, and just evade you in general

Albino Spiders

You know you love them.