IMO, Yasuo and Zed are balanced.
Yasuo is balanced around having a higher than average skill floor and needing to still build at least two crit items for maximum damage output, instead of all those other glass cannons who go around building juggernaut. Zed is balanced around being 75% skillshot-based and being supremely useless in the wrong hands, as well as pretty much every class having anti-Zed itemization at their disposal (
for AP's,
for AD's and tanks are just tanky. Granted, Zed's power level is pretty over the top right now due to the vast amount of anti-AP and ArPen in the game atm.
I do think that Vayne is unbalanced in the sense that there's very little weakness in her base kit to abuse beyond outranging her, and that there's no anti-itemizing against her. I suppose she's supposed to be balanced like Yasuo (relatively high skill floor) but since she's ranged and has free dashes (unlike Yas who is melee and has targeted dashes), she's got a huge amount of margin for error.
If you want to ask me about unbalanced, here's mine:
.
Then there's a whole slew of Champions that are somewhat balanced, but too strong because Riot gives them too much love:
(to name a few).
Again, just my opinions.