What is your favorite questline in Skyrim?
Personally, I LOVE the Dark Brotherhood questline. It's hard for me to kill a couple of the innocents (especially that chick at her own wedding and Gaius Maro) but it's still my favorite BY FAR.
There are twists and turns, badass armor, lovable characters, tragedy, emotion, and a two wizard Sanctuarys! (I've been reading Lord of the Flies. Don't judge my British slang!)
All the other questlines seem so tedious and involve Dwemer ruins and Draugr and blah blah blah. The Dark Brotherhood questline is relatively easy, lasts a good 3-4 hours on its own, and doesn't make me kill falmer or recover ancient artifacts in some Draugr crypt (curse you Greaybeaaaarrrdssss!).
Yes, yes, I'm aware that there's some wizard that needs to get killed in some Dwemer ruin, but it doens't make you go through the entire ruin, and you only have to kill bandits. Also, it's an optional contract. I normally just save it until I feel bothered to get Mjoll's Grimsever or I want to visit Blackreach. I'm pretty sure there's a Blackreach entrance there...
Anyways, yeah! I love the Dark Brotherhood.
Besides that, my next favorite would have to be the Civil War quests. Not because I necessarily like the story or the factions, but because it's just fun to play. Besides the Jagged Crown quest, all the big battles are LOADS of fun. I get to kill tons of people with my warhammer or I can play it stealthy! Plus I get to kill Ulfric (I don't like him much).
The only downside is seeing Whiterun all destroyed and in flames. It's such a pretty city (probably my fave besides Falkreath ((not really a city meeeh))) and it's disheartening to see if falling apart.