1. Orianna
2. Cass
3. Katarina
4. Anivia
5. Janna

Eve, Shaco and Teemo.
You can't play them like normal champions and you need to know want to do if they don't fall into your plan.

Kass is pretty tough to use if the other guy is constantly zoning you. But once you get them scared of you, it's easy mode

Anivia gave me so much trouble when I was new with her now its like cake walk
now i'm learning Orianna I don't think I could have picked another 2 harder champs to learn

Garen
Tryndamere
Master Yi
Shen
Morgana
Kasadin

I thought I'd buy Tryn and cruise, but he's hard to play for me.

I'm only 1440 ELO but from my experience the 5 champs that are difficult to really play well are:

Gragas
Orianna
Swain
Urgot
Katarina

These are probably the hardest to really play well. Obviously it's not hard for Gragas to hit someone with a barrel in a teamfight, but for Gragas to actually get his full potential out, it takes a lot of skill. Just an example.

twisted Fate
lee sin
janna
casseopia
orianna

Orianna
Cass
Anivia
Lee Sin
Karma

NO OTHERS.

Vayne
Xin zhao
Lee sin
Nidalee
Orianna