Lore Battle: Riven vs Yasuo
Who would win in a fight between these two?
vs 
Who would win in a fight between these two?
vs 
We can't really tell. They are both highly trained wandering swordsmen that have fought an won many battles. That is the extent of what we know.
If the context is messed with, like if troops were under the command of either, Riven should have an edge because she has canon experience in this area. Yasuo may as well, but it is not part of his lore like it is with Riven.
After that, we truly can't tell without talking game play mechanics.
big fat sweaty nuts
At first glance, i tought that it would be a close fight... however, then i remembered 2 things: The Blade of Riven is made of Runes (at least 1 one those) and The lore of Ryze says.
"He must retrieve these glyphs before they fall into the wrong hands, for Ryze understands the horrors they could unleash on Runeterra."
So... Considering that, Riven has a sword that can blow up Runeterra. Yasuo stand no chances against that.
Whoever gives each other the most cancer wins
The problem with this kind of question is that we only really have game mechanics to go on in regards to the skill of our two fighters. All we know about Riven's combat experience is that she fought in the Noxian campaign, then broke her own sword. Yasuo also fought in the same war, and was able to beat his brother in a duel. Yes, Yasuo has "mastered" a legendary wind technique, but how powerful is this technique in comparison to a rune-forged sword like Riven's? We don't know, and that's the problem. So little about either of them is shown in the lore and almost none of it has to do with how strong they are, certainly not in any way where the two can be compared.
In short, comparing fluff is like comparing apples and oranges to find out if pineapple is any better.
If we bring in their game abilities and count them as their 'skills', I'd give it to Riven, but only barely. Yasuo's E suggests that his combat style involves ducking and weaving through the enemies he fights, turning their numbers against them. His wind wall blocks Riven's most powerful attack, yes, but that's ALL it would block. Yasuo's CDs are much longer than Riven's, suggesting that he's slower and has less energy than the exile. Riven's highly mobile and has access to a stun and shield that Yasuo simply can't compare to.
However! I said that this would be a narrow victory. And it would be.
What Yasuo lacks in cooldowns he makes up for with an insane damage output. Every blow on Riven would chunk her. If he could get his ult off on her even once, Riven would fall like a sack of bricks. He's also got a longer range than Riven - this seems small given that both are melee champions, but Yasuo's blade reaches longer than most other melee champions (look it up) and his Q is basically a ranged attack on its own, with or without the tornado. And if Yasuo and Riven fought among a group of people (allies or just bystanders) Yasuo's E can come back into play.
Those're my thoughts, anyway ;)
It's to hard to say at this point. Lore wise we have not seen either of them do much. Sure they found some people, but they were fodder. They would need to add more lore. Other than that, I would say Yasuo. If just Riven's sword is magic, Yasuo wins easy because all he has to do is take her sword and she has nothing, whereas Yasuo has technique and multiple styles of fighting. Yasuo would also probably be able to counter any wind technique Riven throws as Yasuo has the skill and training to control the wind, while RIven has an op item and less skill in the wind.
I asked this a while ago in this thread. I originally thought that Yasuo would win, but after reading the responses, Riven definitely has a chance. Maybe she would win.
EDIT: Yeah, I just completely re-read the responses, and Riven would win without a doubt.
My vote, is no one, they would either both be mortally wounded, or by the time the fight is over they'd have grown as people enouhg to let go of what happened in the past, and simply go their separate ways.
the salt generated would obscure the winner.
honestly think riven has the advantage though.
Yasuo, by virtue of having had more exposure due to Taliyah's lore, at this point has superior showings.
Master Yi or Tryndamere would beat them both at the same time though. =)
Riven can't control her anger i think and also Yasuo is a samurai so he wins by default