Ezreal would probably be my top choice for hardest to play.
Actually LANDING your E takes so much more skill than knowing how to wall with Anivia.
Top 3 for me, personally would be something like...
Well.. looking through the champ list almost all of them are pretty easy.
Things like extremely squishy, and covertly squishy (aka seems not squish but is)
Also, things like mixed or confused damage outputs (Characters SIMILAR to mundo, where you have a ranged nuke but are an AD carry but all your damage comes from your AoE on W.)
Confused damage output is what makes a character challenging to me. If I can't figure out what skill I should be using (aka my rotation RQWE, RWEQ, etc) then the champ is a lot harder to me.