User blog comment:BerryBandit/Mechanically demanding champions/Quick decision champions/@comment-3466579-20130623092449

From my experience, Syndra is probably the champion with the hardest / less intuitive mechanics i've played I think she truly deserves the 100 difficulty rating, while i can't say the same for Cassiopea, which, in my opinion, is quite forgiving thanks to the spammablility of her skills.

Another very tricky champion is Lee Sin, with his "flashy" gameplay which requires a lot of cunning and positioning.

Then, let me add a support to the list: Janna. In my humble opinion her ultimate is the most difficult skill of the league to be consistently used at maximum efficiency (i.e. not for escaping purpuses only). Aoe, radial repositioning skill. There are no other skills that can be so powerful, either  for you team or the opposing one!