Ingun Black-Briar for the same reason someone already mentioned; being the only half-respectable member of the Black-Briar family AND the hottest one, too.
Let's see, then there's Jarl Elisif the Fair.. I could think of a few things she could do with those cute, pouty lips of hers (at least she looks natural as opposed to a facebook duckface kinda girl).
Muiri the alchemist's assistant from Markarth. They haven't developed her personality much, but I'm ready to look past that for her sheer cuteness factor.
Ysolda, my wife. Great personality and looks.
Aela the Huntress would be hot, but that voice of hers is just a bit too butch and bossy.
Saadia.. again, as mentioned above, she's a devious backstabbing wench, but meh, I'd still take her.
Then there's that dark elf girl living on a farm in the Rift.. forgot her name, but she was complaining a lot about being bored and wanting to see the world. She was pretty hot! Sadly there was no option to marry her.
Somehow it came as no surprise to me that Brynjolf is getting a lot of love from the female members. Must be the Irish accent and dashing rogue personality.
