League of Legends (LoL) Question: What is the most difficult champion to learn and why?

Everyone thinks differently about this. You can't say that one champion is the most difficult, every champion involves skill in a way, it's just that some champions have less complicated abilities than others. When Kayn was released, I was really confused on his abilities, but I got it. On the other hand, Lux was a quite easy champion to use and learn.

Azir, is hard in that you have to position and aim and kind of be used to moving the soldiers.
Personally, I think a hard champion to understand/play is Thresh since you have to time the abilities correctly, and you can use his abilities in so many different ways.
(Just one difficult champion out of lots of others)

I would personally say any champion that requires hyper rapid action like Yasuo and LeBlance are difficult. Especially if you don't really have the hardware to support that level of reaction required to handle them.

I personally haven't played her yet, but from what im hearing from friends, anivia is an extremely difficult champ to learn.

I am tempted to say Riven and I will not be really wrong in saying it, but I have this feeling that there are more difficult champions, definitely not based on mechanics, but on ways to utilize their kits. Based on mechanics, perfect execution and timing I think hardest champions are: Riven, Azir and Zed.
I might be missing some champion, but meh!

I think that in theory, the correct way to go about answering this question is to consider how many different ways their kit can be utilized in and how difficult it is to actually pull them off. For instance, Champions like Thresh, Heimerdinger, Nidalee, Jayce, Ryze and Sona have different ways to utilize their kits, either because their skills do different things based on how you use them or because they have more than four skils and you need to know which ones you should use at any given time giving you more to learn to properly utilize them in the best way possible every time. I think Azir is kind of complicated, but more because you have to get used to a different auto-attack style and not for any big complexity in his kit. A good examble of a champion that is difficult in this way is Thresh.
Another way a champion can be hard has to do with how dificult it is to lane with him. Without taking counters into consideration, Malphite top has a complicated laning phase, because if your opponent doesn't make any mistakes in his positioning or wave control, you will not have many opportunities to safely farm or solo kill them.
I'd also like to mention Ryze, Kalista and Tahm Kench, because their ulti's effectiveness is completely dependant on coordination, making it either useless or gamechanging depending on the use and the rest of the team.
All in all, I don't believe league has any champions that are actually hard to learn, watching a video and playing 10 games should be enough to get used to all of them, it all depends on how well you know League in general after that.