I was just wondering in your opinion whos the hardest champion to play?

I can't tell you that because most champions aren't "hard." I can however tell you the champions I have trouble playing:
1. Rumble
2. Olaf
3. Janna
4. Galio
Don't know why, I just can't play these champions to the standard I wish I could. =P

Kassadin, Cassiopeia and LeBlanc are generally considered high skill cap characters, mostly because they are very combo based, or have weak periods in the game that you need to be especially skilled to make it through to remain relevant for the rest of the game. Personally, I can never seem to have a perfect game as Shaco or Swain, due to the seemingly cliff-like falloff late game.

Karma. She's fun but pretty hard to master imo

I've found Anivia to be rather awkward for me to play. Ult + E works, but then you have to wall off, and land some Q's as well. It gets annoying to have to do it all at once.

Swain

i would say Anivia as well

Evelynn.

@ScareKrow - Akali, difficult to play? wut.

Initially theres a steep learning curve with Orianna and Ezreal, as you have to learn the positioning and timing from trial and error.

Also, Rumble requires constant awareness of your Heat bar to be played effectively - he's terribad unless you're above 50% all the time (damage wise) I personally haven't put in the time or effort to learn him properly :P

Orianna and Anivia are fairly tricky.
Renekton and Rumble require focus and true understanding of your champion to get the most out of them.

Olaf is hard to play...? New to me. I figured he was an easy champ to faceroll through normal games with top lane.