Ever since completing the Night to Remember quest, nothing has come close to challenging me, not even Krosis. I unleash Lydia and a Dremora while I take cover and watch them take care of business, occasionally using Sanguine Rose more than once if need be, like with Krosis.

So far my toughest "fight" was a Blood Dragon, Frost Troll, and Giant all at once.

I made pretty short work of them with Uhtgerd, Esbern, and Delphine tho.

I started a new game and my character COULD NOT BEAT THAT TROLL!!! Spent ages trying, I was only level 6 or 7 but he just laughed at my battleaxe and swatted me away like a fly.
I eventually just ran past him, turning around occasionally when I had enough room and peppering him with arrows (that virtually bounced off him).

When I got to the steps of High Hrothgar and entered the building I felt safe, but the Greybeards looked a bit nervous. Five seconds later the Frost Troll burst through the door, and I started to run again, but the Greybeards unleashed with Ice Form and Fire Breath and the troll was dead in about 4 seconds, it didn't even make it to the main room, it was still virtually in the doorway.

Those old guys can certainly look after themselves, that's for sure, and totally saved my life.

Giants. Simply because I'm too scared to go up to them. I'll be level 100 and STILL not try to fight them

Two Hagravens in league with 4 witches all at once- They killed Lydia in no time flat, then me when their attention was focused.

I found a cave with a mini waterfall, a falmar gloomlurker and a centurion at level 12 or something. And boy was it intense. I reloaded 20 times to beat them.

1. Jump in face to face (death x 2)
2. Try to sneak pass them to get to the waterfall side. (death x 2)
3. Lure just the gloom lurker to a narrow tunnel and fight (death x 5)
4. Lure the gloom lurker to the edge of water fall, apply only vial of paralyze 5 sec potion, hit him, jump over him and fus him over the waterfall before poking him to dealth with arrows. (tries x 10)
5. Poke centurion with 20 over arrows from the narrow cave and kill him, got too cocky and stood too close to his stream breath and died once. (death x 1)
6. Loot, obtain first filled grand soul gem in my first 8 hours of the game. Happy.

1: Dremora Valkynaz (Those Mehrunes Dagon Trolls you with)
2: Ancient Dragons
3: Dragon Priests
4: Dwemer Centurions
5: Draugr Death Overlords

This is my top 5 list on Master. Now if you have mental problems or play on console with no magic ressistance (hopefully fixed soon). Then the highest level spellcasters you can find are the hardest opponents, my warrior, even with 55% magic res got completely fried by an Arch Pyromancer.

These are the enemies that have absolutely laid me out the first time I encountered them:

Krosis
Thalmor deathsquad
Forsworn (especially the Briarheart variety killed me in 2-3 hits when I was level in the 40s)

Yeah, those guys are pretty tough.

Three thugs claming they were here to teach me some manners.
Oh, snow sabre cat near the Winterhold was also the nightmare for me.

I used to say Morokei as it took me 30 mins to beat him with my mage. But then I watched my friend, as a warrior, kill him in 20 seconds with just a few swings of his sword.

I am currently level 51 and I would have to say that Ancient Dragons are the toughest challenge.

Draugr Death Overlords, kicked my sorry butt the first time I fought one totally wasnt ready for him and his shouting at me. Krosis drove me off the mountain after I killed the dragon had to come back a few levels later and teach him some manners and he still nearly handed me mine on a silver platter. Havent had the pleasure of a Thalmor deathsquad yet but lord knows I and long overdue as I have killed my fair share of Thalmor.

By far the salmon. I've drown three times trying to get them.

Briarhearts. I'm thinking it will be an easy fight as I just took out a few other guys with 1 hit...He took me out with 2.

The two bears I had to fight on The 7000 Steps, on my very first character, at a low level. All I could do was die repeatedly >.>

The Frost Troll I have to fight there always gets me.

Spellcasters like mine seem to have a tough time with Morokei and other enemy spellcasters. I almost used all the potions I had on Morokei. It was the toughest fight I had so far and I was around level 17 at the time. I learned that I did not have to be near my follower to heal them. Knowing this, I healed Lydia and she took him down easily with her two-handed weapon. I hate it when your summoned daedra turn around and attack you.

Master vampires. I never get spell resistance, no.

Spriggan Matron and her four pets

Before that, I'd probably say the Cryomancer. Rotten mages.

Most annoying enemy? Any draugr lord with Disarm. We need a shout that lets us find our weapon again after they've shouted it out of our hands.

My first Chaurus Reaper and its brood under Frostflow Lighthouse. Finally had to lower the difficulty below Adept because I couldn't take hearing their horrific chittering anymore.

Draughr Deathlord and Wisp Mother and Morekai.

All under level 15, those guys hit HARD.

No. You just got to kite them. Use arrows with slow or paralysis. Dual cast Flame. Then run and get your distance. Fire a couple more arrorws until magika returns. Repeat until dead.

alternative: calm and backstab

LOL damn a reason for that spell roflmao... Guess you have to perk that baby up for it to work lol....

I have to admit, even at level 58, Briarhearts are baaaaaddd MF's. Probably b/c they're one of the few npc's to keep leveling with you. There's been more than a few times I wasn't paying attention to the Forsworn I was killing and went up to these thinking it'd be easy peasy to then be killed in 3 or 4 hits. With Lengendary Nightingale armor on.

After it happened 2x, I started paying a helluva lot more attention to what kind of Forsworn I was fighting.