Which champ is more difficult to play? Zed or Yasuo?

It really depends about what you mean as "harder"

Zed has more mechanical tools at his disposal which makes him hard. Yasuo is harder to have a good pefromance with as it plays around mind games, high reflexes and decision making.

I would say definitely zed, his mobility with his shadows is on a higher cooldown, lacks hard CC, and doesn't have a defensive tool against ranged nearly as potent as yasuo's windwall.

zed is way harder to play correctly

zed.

Yasuo... Ever notice how easily it is to feed on him?

I find both hard to play in different ways.

In this meta i would say zed since it is more difficult to make him work in the already chaotic teamfights you have to manage your shadows well enough to get into their backline to get a kill, on yasuo you can just hit r to get onto them and land a few autos or q's and most of zed's damage is through a skillshot which is not too hard to miss, while yasuo can miss q's but still have decent dps without it, in a 1v1 lane i think yasuo is more difficult though because your e isnt a get out of jail free card, if you go in you're risking death if the jungler just happens to be around, but on zed you can sit back and throw your combo out untill they get low enough to kill with an ult, but then again teamfights are more impactful to a game so yeah zed is harder.

Zed,he can't be played by monkeys lol