I just finished Forbidden Legend and I lost my special horse and lydia...and one of the bosses kicked my ass all up and down the field.

Can't really say I've had a hard quest, just moments in quests which were difficult at first because I didn't pace myself or use sense. Like during the MQ when you go to Skuldafen, the 2 dragons greeting me+various draugr overwhelmed me. I couldn't fathom how I was supposed to beat that combination, when I could barely (at the time) beat one dragon. I got around it though by learning to draw enemies out bit by bit and goad some while others ignored me.

The dragon priest near the portal to Sovngarde was my first encounter with those draugr type as well and yeah, that svcked until I realized I had to go for full offensive power and just relentlessly slash away to win.

The Boy Who Cried Wolf... or whatever the second quest is you do for Solitude. Draugr of all levels everywhere plus Potima the shocking ghost. If you do it with a follower it's not so bad but doing it solo is hairy.

Skuldafen deserves honorable mention but as already stated, if you know how to draw out the enemy one by one or even in pairs, it makes the thing a lot easier. The ending to that quest before Sovengarde was tough too, until I realized that one decent invisibility potion solved all problems. You can't do that with the 'Wolf quest. You have to fight the horde and then Potima.

And if you decide to test out J'Zargo's scrolls right there, like I did ... yeah, you're majorly screwed over. I'll pay him back one day, mark my words ...

J'Zargo's Experiment. Every time I walked into a Draugr with the Scroll on I died. In the end I fooled around during his training in the Hall of Elements and got him killed.

Forbidden Legend for me too. Because of the difficulty my character is using the amulet and always will.

Forbidden Legend, seconded. (or thirded? is thirded a term?)
I've heard that Arniel's Endeavor is supposed to be one of the most complicated quests though. Can anyone confirm this?

Wasn't even all that special of a quest, just killing some bandits in Treva's Watch. Several bandit chiefs, archers with 1-hit kill accuracy, and mages. With traps that are difficult to get your follower to avoid, that make noise and aggro all nearby bandits.

Getting into Blackreach. Spent at least an hour and a half fighting a Falmer Shadowmaster and 2 nightprowlers, dying dozens of times.

The Staff of Magnus

I did it with my Permadeath mage, but it was a major pain.

The 2nd Potema quest in Solitude
Sheogorath's quest (Because your unarmored and that can be tricky at times)

Khajiit found the retrieving of the... Jazbey Graqes... to be most horrible. So long has J'rahzir walked, searching, stalking, and hoping for such graqes. Khajiit is foreign in Nord lands. He knows not where the graqes rest.

Not complicated, just time consuming. Still fun to have a spell to summon his shade.

I'll third the 2nd Potema quest.

I had to use like 25 health potions for the boss portion of that quest (thanks mostly to the fact that Potema's lightning kept draining all my magicka so I couldn't use heal spells). And I got disarmed like 5 times or more, having to restart the battle twice because I couldn't find my disarmed weapon. Actually, I lost my weapon in my final attempt as well, but I'd gotten so annoyed by the battle that I just said "Forget it, I'll live without it" and went on. Thankfully it was just some generic weapon and not anything special.

I can't imagine doing that quest as a mage. It was bad enough being without magicka for healing on my melee character, but not being able to attack at all due to constant magicka drain would be downright deadly.

Skuldafn because EVERY 5 MINUTES I WASNT AT SKULDAFN A DRAGON WOULD ATTACK ME!!!! plus the deathlords and dragon priest

Getting Spellbreaker, I hated the projectile vomiting
Deciding to pick on the two dragons gaurding the portal to Sovengarde in Skuldafn with a level fourteen character, I can kill one and then view the CHOMP as my character enters Sovengarde...

Wander in the steamsprings area.... LOTS of graqes there.... south of Kynesgrove.... Also, at least 8 Jazbay in Skybound Watch - mostly in the northern end.

I will 3rd that, I went in there ill prepared got my tail end handed to me and still have yet to go back and get around to finishing it.

Khajiit should take the word of a Bosmer? He disagrees. Graqes are not strawberries. They will not grow too near to danger. J'rahzir has called the Bosmer's bluff.

This Bosmer has spent many many days exploring the regions of Skyrim. It is an interesting country. Full of Nords who have no couth, and Khajiit who are friends (because this Bosmer has helped at least one with something lost now regained).

Serethil (who is Bosmer only in this incarnation) would ask J'rahzir to simply explore the areas noted. If J'rahzir finds nothing else, he will find graqes. He will also find adventure, and loot.

What is this "strawberries"? Now, to Serethil, if she is to take this word literally, this is straw piles with dung ("berries") from the latest beast to use the bedding....

J'rahzir knows the berries of straw to be popular in Cyrodiil... Though, it goes against all that J'rahzir has learned, perhaps he shall find a berry of straw for the Bosmer. Though, the Bosmer may need to maintain such a polite manner and be correct concerning the graqes. J'rahzir is still skeptical, though the shock of never tasting such strawberries particularly shocks J'rahzir. This Khajiit feels the need to remedy this indeed...

Serethil would be pleased indeed to see such a berry, and to taste same - if said berry is NOT simply one of the leavings in the straw from beasts and men in this cold, unfriendly land....

This Bosmer does gently request that J'rahzir consider a small trip to the areas aforementioned, in search of said graqes....

I haven't done the Potema quest yet, so I can't say on that one. My hardest quest was "The legend of Red Eagle" It probably isn't as hard as some, but I went into it at a pretty low lvl. Got my backside handed to me waaaaay to many times.

The Sheograh quest.... I got sooo mad on the first part I did, that I stumbled on the right answer.

Then, J'rahzir has agreed to such a challenge. He shall return shortly. *three game days later, J'rahzir returns, smoldering* So... J'rahzir found graqes. He shall avoid graqes from now on and find a fine strawberry for the Bosmer. Later... *coughs smoke*

Forbidden Legend. Some of those bosses where on freakin pcp or something.