If you’d asked 15 years ago the answer would have been Kaylee without hesitation, but after seeing how Morena Baccarin has gotten hotter and hotter as times goes on I have to go with Inara now.
The ladies are all cool and beautiful in their own way, but Inara would definitely be my pick.
None of them hit me in any particularly special way, but Zoe is the one I’d ask out to hit the town with. I like being the silly one.
In world? Kaylee is pretty much the only sane choice. Maybeee Zoe, but probably not. She’s like Mal, still fighting a war they lost a decade ago. Lost and unwilling/unable to admit it. There’s real PTSD there. The mystical sexuality stuff turns me off for Inara, and it’s literally the only life she knows. River is just too damaged to be a decent partner, and canonically she’s ~16( to be precise she was 16 when Simon began trying to get her out of Alliance hands, not sure how long it took him, but probably <2 years).
But the whole Firefly 'verse seems like a special kind of hell to me, so I based it on the actresses, not the characters. And Summer Glau takes it in a walk.
Enjoy,
Steven
Kaylee over Inara by the slimmest of margins.
I voted Zoe, but only because Saffron wasn’t a choice…
Kaylee, with Zoe second.
It is like you have never even heard of Joss Whedon.
Kaylee. With a strawberry.
I have to go for Shepherd Book. But who could have any relationship with River other than “try to protect her”? She’s a damaged child. A terrifying damaged child.
Sounds like we need a Mal/Jayne/Wash/Book/Simon poll. Maybe later. 
It’s been a long time since I watched Serenity, but my recollection is that by the end of the movie, she’d moved beyond that. And even during the brief series, she’s making progress. Still damaged, but clearly more than just that, by the end of the series.
Kaylee is good-looking, but also incredibly sweet.
Inara is stunning, but the courtesan puts me off a wee bit.
River is also a looker, but (not her fault) damaged.
Zoe is pleasant, but too tough for me (again not her fault.)
Kaylee is my jam. Inara is out of my league. Zoe and River are intimidating.
Damn, you’re right - I should have definitely included Saffron! How come I didn’t think of her?
You win the thread!
I am a leaf on the wind.
Inara, hands down (or up ?)
Years ago while I was watching kids at recess, a boy, running giddily past me, shouted this line.
I ducked.
Why? Are you a [del]reefer[/del] reaver?
Kaylee, especially if it’s been a while since she’s had something twixt her nethers.
Or Yolanda.
Or Bridget.