A hard choice for myself... considering so many variables. Such as players ability to be able to land various skillshots... But personally, for myself the list is;
Lee Sin: A good Lee is quite able to do things no other champ can, and if he really is that good it will be very hard to lock him down. The Second part to all his normal skills make him one of the most versatile champs around. But the fact that you actually have to know what you're doing with him makes him pretty hard/
Orianna: A very overlooked champion. She takes a serious amount of practice to get down just right, like knowing her exact range on every single skill. (I myself have to work on timing my ult just a bit better..) She excels as AP mid or a Support, so there is alot of difficulty between the two roles.
Nidalee: Let's face it, we've all faced Nid's that have just wrecked our face with those spears. And when said Nid has 700+ AP those spears hurt, alot. Her ability to function in both Human and cougar form leave alot of options available, plus her Attack Speed increasing heal? Lets her fit almost any role. Each one takes a different amount of skill to perform properly.
Karma: Firstly, who? But secondly, Karma is one of the more versatile supports out there, but she isn't just a support. I've seen a couple karma's go into mid and just dominate it. With her Unique Mantra system she takes timing and skill when to know how to drop one charge for a skill.
Anivia: Ahh yes, the famous blue pigeon. Anivia is a heavy burst champion that alot of players underestimate, understandable if the person in question is not too good with her. But a good Anivia is quite able of putting most other champions to shame, her Q is an AoE stun, she has quite a large wall, her single target nuke that melts faces, and her flashy ult. Each part of her making for interesting combinations in a team fight. Quite interesting but reliant on ability to land her stun.