For me it was Potema. I really felt like a mythic hero. I'd been reading about her forever in every TES.

Morvath was cool also but I really wanted him to kick my ass in hand to hand if he got the first blow like in Immortla Blood not magic me .

I have to say that the end game was seriously disapointing. It was like: I saved the world and i get 0 praise?
I would have to say complete the thieves guild. It was so boring to do those mini quests things >_>