What quest, in Skyrim or its add-ons, did you enjoy the most and why?

For me it is without a doubt Lost to the Ages.
What I liked the most about that quest were its mechanics.
For starters I enjoy quests that start by reading a book. There is something statisfying about 'discovering' a quest on your own like that as well as that a book can give more information and background than you usually get from an NPC.
Furthermore I enjoyed the discovery present in the quest.
If you havent visited the locations yet, there will be no quest marker pointing towards them.
You have to read the clues that youre given and compare maps. Then when you get close the arrow pops up. If however you already know the location there is a quest marker and that makes sense.
I can almost hear my character going: "Wait a minute, I know that place!"
The puzzles were interesting. Not deviously hard, but not immediately obvious either. Looking around for clues helped.

The rewards were unique and thats always good, but as well as looking nice they were also really useful.
The ability to have two standing stones active at the same time helps me a lot in cutting down on grinding. I can have a boost from the Mage stone as well as the Lover when raising my enchanting skill and I like that. Also great that there are three unique rewards and you can only chose one.
It urges you to choose carefully and lends a bit of importance to that decision, its not just something that you take from a chest.

And lastly the story itself.
The ghostly companion relying on you to finish her life's work was nice and she had an interesting personality. I especially liked her various comments when you both were making your way through the ruin that begins the quest.
Very statisfying as well is the epilogue where you confront the man who stole her research and tell him you know what he did.