Probably one of the Dragon Priests.

An ancient dragon, blood dragon, and named dragon priest all at the same time.

This is true story:
1st Giant Frostbite Spider on the first dungeon (G*lden Cl*ws)

Yep... i dont know anything yet about skyrim, followers and many else...

so, i just keep rushing through, and this bastard poisoned me, and i was died

This. Draugr Death Overlord, since my character was a melee twohanded warrior.

On level 3: Sabre Cat
On level 10: Giant
On level 15: Draugr Scourge
On level 20: Draugr Deathlord

Personally: Havunraak

Drela and a Draugr Scourge Lord, which was harder than the actual boss fight of the dungeon.

I handled them ok with my sneak archer and my mage, but my illusion/sneak/dagger elf had a hell of a time with one early on. It wouldn't have been so bad, but the damned thing happened to draw the disarm shout, so no matter what I used, as soon as it got within 20 feet of me, it would blast my equipped weapon off into some dark corner where i couldn't find it anymore. Spent 20 minutes running around up one set of stairs and down another, round and round, with that undead ***hole chasing me like a godawful shouting boogeyman swinging a sword the size of a lamp post, half the time being disarmed and desperately searching my inventory for some damn rusty butterknife or spitwad shooter to try to injure it with... finally took him down with some craptastic hunter bow or something and mickey mouse arrows. Took so many arrows to finish him off, I leveled up my archery from abysmal to almost semi-crappy.

A cave bordering the sea up north--wandered it. Made it through part way and bandits struck our group--I was beheaded twice, not sure if it was a boss--didn't live long enough and didn't stay around to find out.

Forsworn Briarhearts, Dwemer Centurions, Ancient Dragons, Dragonpriests, Draugr Overlords, Hagravens, Draugr Overlords, take your pick. The first time you encounter them, you're like "oookay... what am I going to do now...?"

Things get a lot better as you level/gear up, but you'll never forget that first time

the dragon priest in the main quest on a sword/shield character

I've got to add the first time I ever went into a Falmer cave, too. This was on my level 4-or-so Redguard archer. I've no idea why I did that, but I was hell-bent on making it through. Let's just say it was mildly impossible, since the majority of the Falmer there were ten times harder to kill than your regular standard-issue bandits or draugr.

I never had to much problems with Dragon Priests, but that may be because I usually have the advantage of sneaking.

I had great trouble with Red Eagle on my level 10 sword-n-board character. Out of the 10 times I had to fight him over because he kept killing me, his first action was to disarm me, after which I could not find back my sword and shield. Eventually I turned my difficulty down to Novice, and even then it took me a few tries to kill him before he had the chance to disarm me.

Drelas, the dude hiding out in his cottage north-west of Whiterun is pretty hard too. On Adept, I never managed to beat him when I was under level 40, since his health is pretty high and he keeps healing.

I made the mistake of wandering into Shearpoint while i was at a low level, Krosis taught me a valuable lesson.

Bandit Marauder mages. Insane damage for such a generic foe.

Oh my god, ever fought a Arch Cryomancer as a melee? It's a frickin' nightmare.

Arch-anything are just completely ridiculous. It took me (100 in destro, alt, conj), Marcurio, his Dremora Markynaz, and my two thralls (Volkihar MASTER Vampire and Forsworn Ravager archer) to kill one Arch Necromancer. I immediately thralled her and checked her stats; 675 health, 500 mana, 55% magic resist, all magic schools except illusion were higher than 85. She is a monster. Ironically, she svcks as a necromancer because the enemies we face are usually too high for her to revive. Incredibly powerful mage, though.

EVERY ARCHER IN THIS GAME IS AN EPIC BOSS FIGHT.

Hired Thugs on Master. -__-

^{The gang that loiters around skuldafn temple. You know the bunch. Nahkriin, his two dragon friends, and his little entourage of draugr deathlords.}

How so? I either pick them off easily with my archer, or charge them with my melee chars and hack them to bits. While they can be annoying in numbers, they can usually be picked off by tactically retreating and drawing them into an ambush spot.

I know you can't kill him but the first time I met Tsun (sp?) I really struggled. Had to keep re-loading time and time again, until I realised he doesn't like getting his feet wet, but even then the fog still made him difficult to hit from distance.

Draug Deathlords can be a pain in the proverbial, especially when there are three or four of them together.

Yes. I spent about 10 minutes in my first battle and had to run and heal a bunch of times. I love the masks though.

I took over 250 points of damage on one shot from Sigdis Gauldurson.

Yes, some of the bosses with bows can pack a punch, for sure. But that hardly qualifies as "EVERY archer in the game is a an epic boss fight." Most archers are quite easy to kill and don't hurt you all that much.

Rahgot or Krosis.

Dragon Priests are a pain, but they're worse when they fight with friends.