If I could include the Dawnguard DLC in this, it would most definitely have to be finding Auriels bow in the Forgotten Vale. There was beautiful scenery and beautiful music. But my favourite moment in that quest was when I was walking over the frozen lake, and the two dragons Voslaarum and Naaslaarum broke through the ice and tried to kill me. It was a fun battle indeed.
Overall, a very enjoyable quest.
A good quest always gives you a Night to Remember... geddit?
*Ahem* I like any quest which requires any skill, such as the Thieves Guild quests (shadowing and infiltrating), Blood on the Ice (Investigation) and Lost Legacy (puzzle-solving).
Also, there's some satisfaction in becoming the Lord of a court of vampires. Just saying.