Farkas? Facars... Farkas?
Since Serana can't be married, my favorite is Sylgja. Her story and the fact that you meet her parents and help them makes it extra special.
I don't know the male NPCs too well, but either Roggi Knot-Beard or Scouts-Many-Marshes. As you can see, I like me some minor characters.
One of my female werewolves married Aela, and it actually destroyed my game. When she complained about me moving her to Lakeview Manor it seemed so out of character that I couldn't stand it. I like Farkas the best, for my Argonian and Khajiit characters I like Scouts-Many-Marshes. Aela has been my only female choice, so not sure about other female options. But Sylgja does seem like a good pick.
For me it has to be Muiri, something about marrying someone who had you kill their ex spouse, then killing the emperors cousins wife to get her proper wedding garments just says 'everlasting'. haha. Or ysolda, she doesn't have annoying dialog, so long as you can accept her underground trading habits.