River. Hands-down.

My darling psychic would kick Buffy's a** something fierce! :) Buffy would never even know what hit her.

That is, if Buffy had not already collapsed under the weight of River's exceeding beauty and elegance, before the fight ever got on, of course. :) But assuming she made it that far, the next scene would show us Buffy, on the floor, spread about with all her buddies, motionless, and River just saying: "I swallowed a bug."

Well, that is what I think, at least. If you prefer to see the event alone, without bias, I suggest you ask the Operative. :)

I actually think that Buffy has the higher endurance levels, she's THE SLAYER! Although I definately agree that River is a way more skilled fighter - she's super fast, amazingly coordinated and of course can read minds - but don't forget - only Buffy is the one with super strength and endurance.

Whatever else she might be, physically River is only a human. Were she to receive any serious hits from Buffy with her Slayer strength, I think River would be the first to weaken. Buffy has far more endurance, she would just have to be extremely quick to overcome River.

But to continue off on a fan tangent, I can't see these two fighting eachother, they'd probably be on the same team, unless River went evil.... In which case Buffy would have to slay her.

Honestly I love both these characters, partly because below the "tiny girl with superpowers" surface they're completely different :)

I'd say River would win and you know why?

Jayne would just hit Buffy over the head with a pipe or something.

River

Yeah, I pretty much thought River too... but, you know, you just have to ask these questions
and it's fun to pass the time .....

River would definitely be victorious.

She can kill Buffy with her brain. Buffy can't claim to be able to do that now can she?

In an addendum...well, let's lay it out...
Buffy...augmented strength, heightened healing abilities, the occasional prophetic dream. Also, trained killer.

River...Reads minds, trained killer.

Yeah...I'm going to stick with what I said...But, In a straight out fight...Buffy would wipe the floor with River...

I'm gonna go with River, but only because she would actually KILL someone. Buffy has fought mindreaders before...I mean, I think she could take River, but, I think she would hesistate, and River would go to town...

So...Buffy COULD beat River, but, River WOULD kill Buffy...

And, yes, I've finally handed over the keys to my brain to big f_'n geek that lives there...

In answer to your title question, Yup, this was asked before but don't worry I never get tired of answering it!
In my opinion, River would win. Here's my reasons why, River is much quicker and more exact with and in her movements, Buffy is a little sloppy in my opinion, River would see this and use it to her advantage. Also, I believe River has a higher endurence level and she would eventually wear Buffy out.

River.

She can read minds.