Hardest Champ
- To me the hardest champs are Cass, Zyra, Elise, and Syndra. They all have skillshots that are very hard to land and have very hard mechanics to learn.


Syndra and zyra aren't really that hard compared to other champs. (at least not for me). I've always been able to do well with them, even the first times i've picked them up. I would add Kassadin because along with elise and cass, you have to know how to play them to do well.
Lee Sin.
"Master yourself, master the enemy." 
Azir, boom. Not even Bjergsen could get a 50% winrate with him.
In terms of playing a normal or ranked with (taking into account all match ups and builds and what not, as well as doing your job as that class)


It depends on your definition of "hard."
If by hard, you mean requires you to have superior reflexes?
or
If by hard, you mean requires you to spend many games to get the hang of it?
or
If by hard, you mean requires superior positioning skills?
or
If by hard, you mean requires you to calculate your actions to a science?
or
If by hard, you mean requires you to always use your abilities correctly?
or
If by hard, you mean requires you to have excellent map awareness?
or 
However, none of these definitions determine what champion would be the most challenging to play. That kind of thing varies from person to person. I've known plenty of people that could play a champion like Lee Sin or Fizz very well, but would fail at life when trying to play Volibear or Jarvan IV.
Depends on one's definition of hard. There are many different aspects to that concept.