I'm wondering what are the most difficult champions to play with? Which one of them are worth the effort, too?

I vote for Anivia. If played by an unskilled player, she can do more harm than damage. Wrongly placed walls are the worst, but bad skillshots (that big chunk of ice) and not knowing how to combo her spells are also awful.

For some reason, I play her extremely well (almost never die) even though she is considered a very difficult champion. And I fail at some much easier champions, for some reason.

ryze is pretty easy actually. your late game is super stronk and you practically faceroll teh keyboardz.

i would say anivia since most people cannot hit the Q and they cannot place the wall perfectly like that froggen dude.
orianna too for pretty much the same reasons, positioning that ball isn't as easy as it looks

It's practically impossible to die with Anivia during lanegame.

-MEOW

Darius. My finger gets sore after pressing R all game.

But seriously though, I agree that Anivia has a pretty high skill cap.