... Caught in the Hunt probably has the best storyline for a single quest. ...
I would agree with this for single quest.
I would have to go with The Collector, Nothing You Can Possess, and Secrets of the Ayleids.
Similarly, this takes top honors for a series of quests, although I adore Mazoga's quests as well.
As far as a big questline, I have to say the Thieves Guild - because ultimately it's a nice love story. :wub: