Kaylee has that whole "I like to get fucked raw, yet I'm somehow still sweet and innocent." vibe going on.
River would probably stop mid hump and explain the mechanics of the act, then when she gets to the concept of "Penetration", stick a finger in your eye and say "PENETRATION, SEE?". Then she'd giggle, get up and eat a Snickers bar without a second thought, and start running around the room to see her clothes flap around behind her, leaving you minus one eye and plus two blue balls. Also, it would probably be your Snickers bar because she's a fucking freeloader, so minus one of them.
Inara - While I don't disrespect the profession, a sex professional isn't where you go to stay, you know what I mean? Too clinical and too much traffic for me. Also high maintenance.
Zoe - Was married, now freshly widowed. Also very bitchy and badass in many ways. Me likey, but too much baggage.
Kaylee by a landslide, with Zoe as a very distant second.