Muiri. Too bad you have to be in the DB to get her quest.
I specifically decided against Muiri because of that quest. Without giving any spoilers away, I would seriously hate to get her mad, and would not want to know how that brain thinks. She took things a little too personally and went too far in payback.
I've been trying to decide who to marry as well. I am thinking about Ysolda in Whiterun, and moving her to Solitude or Markarth.
I think Karliah or Katariah or whatever the bloody Dark Elves name that is in the Thieves Guild end quest is awkwardly hot. I think it's her purple eyes that do me in. That and the fact that she looks 25 and is probably 100.
Karliah should've been an option as a follower after beating the Thieves Guild. I am pissed that she just stays around the room where you meet Nocturnal and does nothing, she doesn't even go back to the Nightingale lair.