Got sent to Ansilvund at low level to collect a book. Die, Reload, die, reload, rinse, repeat. Ended up getting them to chase me all through the last level, and killed the big bad boss woman by reactivating the swinging blade trap.

That was the hardest fight I ever had so far also. I died and reloaded six times devising stategy and a lot of poison potions on a glass bow, used a lot of restore health potions and had Volundrung hammer at the end before I killed all that mess down in there.
My follower Areana Ienith died in that fight, and there was no way I was going to reload after finally killing them and it broke my heart!
That was the toughest so far. I can't remember the name of that place either...and the boss had a name I can't remember.

With a sneak skill at 92 and archer at 83, there are no problems in any dungeons anymore. Not been for along long time either.
But I think in the beginning, I had some encounters with Draugr deathlord + a Falmor in a Dwemer ruin, thay caused me a lot of re-loads...sigh..
They were not in the same ruin, though, but you guys understood that

Oh yea. I had one like that. Some big guy with his own Shouts took absolute ages to kill. He killed Lydia who I really liked, but like you, no way was I going through that again.

I ran across a certain dungeon not too far from Whierun at about lvl 30 independent of the quest associated with it. There was a bandit chief in full steel plate armor on who comes through a waterfall. He killed me almost immediately, even though my health was good. I finallt gave up. I'm at lvl 43 now, but I still haven't gone back to try him again.

1 dragon priest and 2 dragon, as a vampire, in broad daylight.

Or the mage i fighted while inside a soulgem.

Somewhere in a snowy mountain i fought a dragon, after consuming most of my potions i saw a word wall > cool, lets learn a new shout. Next think i know the door of a coffin flung into the air and a dragon priest with a meteor spitting staff gazed on me with his fiercely blue eyes. After a lots of reloading due to death i swore to return.

SIghtless Pit

Can't remember the name of the dungeon, but I had a lengthy fight with a Draugr Deathlord who mixes FUS RO DAH with ebony arrows. Each hit would take about 60% of my health, so there was a lot of dodging and healing on my part. Finally maneuvered him in position to FUS RO DAH him into something like a well shaft. Trying to outshoot him with steel arrows was out of the question, and I wasn't sure I could reach him for close-quarters fighting before taking an arrow in the knee for the kill. I was just glad to get him out of the way. There's also a dungeon near the College of Winterhold where I had to use a similar strategy with a Deathlord. Knocked him off the mountain, got my treasure, and fast travelled out of there when the coast was clear.

Dragon priest in the college related dungeon

Gotta love the "Tower of Strength" perk in Heavy Armor. It makes getting hit by "Unrelenting Force" a joke. My Nord Warrior has it, and he can take a full "Fus Ro Dah" to the face and barely even flinches. Doesn't interrupt his blocking or attacks at all.

Hmm...

So far, I'd have to say that fight with the Skinner during the Companion questline. So far, my warrior doesn't do too well when trying to block attacks from the big boss while 3-4 minions are swarming them.

After reading the previous posts, suddenly I don't mind if my Nord feels near invincible.

It didn't take too long for my Oblivion character to feel overpowered, but in Skyrim, my similarly-built character feels like such a weakling.

You know as I read these, it just reinforces my belief that Bethesda has the leveling wrong. What I mean is that you end up playing and everything is fine and then suddenly, you're totally overwhelmed with WAY more powerful enemies than you have been seeing. Maybe normal battles should be harder and the bosses shouldn't be so shockingly powerful.

I'm hoping that my smithing/enchanting/alchemy will let me meet those ridiculously powerful bosses on a more even setting. And I'm saving a perk or two in reserve if I get into something I can't handle (so that I can put the perk into for instance not being staggered if that should prove necessary).

Many times, I'll be breezing through a dungeon and all of a sudden my Nord warrior gets surrounded by 3-4 enemies at once and dies. Even with dwarven/orcish gear, ~250 health, etc. (lv 15), he can turn to glass in a hurry. This is a 360 from Oblivion at the same level.

There are actually same NPC's with varried skills, when I was doing the nighbtare quest in Dawnstar, I was breezing through the enemies, I up a door, and I get a steel warhammer to the face, kills me one hit, so I reload, try it again, Samething, So I tried it again, only this time I was ready, he still owned me, It took another 3 or 4 tries to kill him! And he was just an Orcish Invader like the others were. So some NPC's are just built stronger then others. I love the leveling in skyrim personally, you never know what youll run into! (Though there needs to be more creatures, im sure thatll be fixed after CK release)

azura quest.

I was going to go in there, but the look of it and the name made me think... "Maybe another time..."

It sounds seriously dangerous.

As I was raiding a cave infested with necromancers, a book was on one of the shelves called "The History of Red Eagle". I decided to do the quest, and I found Red Eagle to be one of the hardest fights ever. I was able to beat him, but after constant Firebolt, Run, Rinse and repeat.

I don't remember the name of the dungeon but it was on my explorer character who was at lvl 14. The premise was I had to go find three vials. So once I found them I would just run in and grab them while the boss for that room was waking up. Of course the problem is they chased me but it was kind of mazey with a couple places that doubled back on themselves so I just ran in circles until I lost them. I get the three vials and the quest giver wakes up the BIG boss who uses lightning attacks. I decide to come back another time but I ran the wrong way into the dung eon!

So of course the 3 bosses I didn't kill are all in there waiting for me. I ended up with a mage who had a paralyze stave, two Draugr Death Lords, one using the disarm chant and the other the unrelenting force. So not only was I being disarmed, paralyzed and thrown around I still had that lich shocking the hell out of me. What is amazing is I still actually made it to the exit but they killed me right before the load scene so I was in the death loop reload. I had to shut it down completely and go back to an earlier save.

Skuldafn Temple bar none

yeah theres your problem
On master mage evry boss usually takes a few tries and many normal enemies are the same
Don't get me started on Krosis

That guy you mention wears a different helmet to the 'false' versions of him, this is how I spotted him and didn't fire on the wrong one.

To add, the toughest battle I've had was actually over the quickest. My Level 45 nord stumbled into Volunruud to find the source of power and there was a Draugr Overlord with Ebony arrows that took my health down to about 10% after one hit, he actually killed Jordis, my companion from Solitude instantly! I had to reload and tell her to wait outside and sneak attack him as much as I could along with Unrelenting Force when he got close.

I also got creamed going into Silver Moon Camp by firebolts, I didn't even see who was throwing them or where they were coming from, I was dead in a few seconds flat. I have yet to return there and get my revenge.

I'm only level 15 - stuck in Valthume right now. The death lord was tough, but now I'm fighting this floating dude who is unfazed by my axe....

Many levels ago, I accidentally found lower steepfall burrow while looking for northwatch keep. Got by the first troll, but not the 2nd one. I turned around at that point.

I won't say where it was, but I was doing a rescue mission. As I went through the place, most enemies were met and handled. However, there was one room where it seem liked no matter what I did, these 4 NPCs would not die. If I had not had a bunch of powerful healing potions I would have never made it through there. I finally had to single them out one at a time and wail on them until they died. Then move on to the next one.

That sounds like Shearpoint to me. Luckily, I was level 45 when I went there, easy victory.

On topic, my hardest battle would have to be Forlhorst. I managed to get through the dungeon(which took ages) quite easliy, the Dragon Priest and all his Dragur friends however, not so much fun. I think I reloaded 6 or 7 times for that one at about level 35.