Yasuo has tons of mobility, a wind wall that blocks ALL projectiles coming from that direction(even underground ones, wtf? 
), and can easily make it to 100% crit.
Zed I personally don't think is OP, just toxic to play against, so I'll leave that to the people who do think he's OP.
Fiora can beat almost anyone in a duel and even 1v2 or 1v3, blocks all CC, has several mobility spells for speed boosts and dashes, two slows, and if CC'd can CC anyone else in her parry area. Not to mention if she finishes her ult or kills the ulted target her and all allies in the area heal for up to 700+ health(granted that's only till rank 3 of ult which takes till level 16, but even at lower levels its still pretty high especially with her AD scaling on it).
Vayne is one of two ADC's with a non-ultimate CC ability(along with Draven, but she gets a stun and knockback whereas he gets a tiny knock to the side and slow), every 3 attacks can't be defended against, synergizes extremely well with BotRK and LW making it just that much harder to defend against her, has consistent damage and has one of the highest ADC's DPS in the game and its consistent, a dash on a 2-8 second cooldown(depending on when she uses the AA buff), and an ult that gives her invisibility(easily countered but basically forces you to spend money on pink wards or oracle's), and just that much more AD to make her even stronger. Don't even try to argue that Vayne is balanced.
Ahri is safe as fuck and bursts like an assassin from range.
Riven, after pretty much boots of Lucidity has almost infinite dashes, can unleash her kit within a second(basically as fast as her W is long, and then she has her knockup), and an ult that gives SCALING AD(unlike some AD buffs so its even more) and pretty much after level 6 can beat most anyone. The biggest downside is without Black Cleaver she's squishy, but even then she has her like 5 second cooldown dash-shield(and it is a big-ass shield -_-).
The reason they're played so much is because they're strong af and fun to play. Not one or the other but rather both.
Unfortunately we hate them because they're toxic af to play against as well.
Have you ever thought that these champs are popular and are picked often not because they're "OP" but fun to play? I love these champs because they are flashy, complex, and fun to play I don't play these champs just to win. I hate binary and simple champs such as 
Even if they are good at the moment or not because they're not fun to play for me. When I go supp I mostly play Thresh(mastery 5) because he's fun, I mostly play Vayne when I adc because she's fun and has tons of potential, I main Riven because there is alot you can do with her kit and her full potential is expressed by the player. Some of us players don't like simple champs and are excited when we see new champs with complicated looking abilities and disagree that champs should be made simpler.
