There's different kinds of skills.

Nidalee and Lee Sin require the most practice and learning just to get to the same moderate competence you would with other champions, simply because they have 7 skills, all of which should see use, which requires paying a lot of attention to a lot of things. I'd give the edge to Nidalee here, just because her skills have more variety to them (support in healing and attack speed boosts, support in constant trap laying, harass with spears, chasing/fleeing with hops, and of course actually attacking, including AoE swipes or single target bursts). Not that Lee Sin is simple, but his skills are mostly combat and/or movement oriented. Still, he's also got support skills and is complicated, and needs to enter melee almost always to do his thing, unlike Nidalee.

Some champs require watching the entire map and knowing what's going on everywhere at all times to play perfectly, which is pretty hard to achieve, but it's also a skill that transfers between all of them. This pretty much includes anyone with a map wide effect: Ashe, Ezreal, TF, Pantheon, Shen, Soraka, Karthus. Of course, this map-wide-awareness skill is mostly the same skill that anyone with teleport should be developing, so in some ways it's easier to acquire this skill, and partially mitigated by anyone playing on a full team in voice chat. Of these champs I'd probably say Shen is the toughest to master, between timing and aiming all his skills and doing energy management (which isn't as much of a thing on any of the others).

It's a pretty hard thing to judge overall, especially since everyone starts with different people and some things are just harder for some people than others. I couldn't say that Shaco or Eve or Anivia or anyone else is just a wrong choice.

steepest learning curve, IMO is Nidalee

Shaco Nidalee and Anivia have the highest cap...all three can be dominating when mastered.

It is not too difficult to do 'ok' with Anivia, but there is a lot of room for improvement (high skill cap) for that champion.

The wall can be an all or negative-all skill-shot that takes patience, map awareness and remembering who has their flash up and who does not. You can totally separate the other team, or isolate someone on your team in with theirs. There are a dozen or so tricks to use with the wall, and most of them are difficult to pull off and require good positioning.

Her Q is a double skill shot, one to aim and another to detonate for the stun. If you factor in the it's slow movement, it is a slightly different skillshot than other champions with similar casting ranges (Lux, Morgana)

Her R is similar to other 'circle on the floor' skills like Morgana, Nasus and Malzahar. It is not always easy to get it in the exact correct spot for team fights (though probably the easiest part of her play other than the targetable E)

Cass>Nid>Karma>LeBlanc probably.

If you think skill shots are hard...then you probably have a different set of criteria for "hardest".

Cass doesn't have traditional escape options, and doesn't burst like other AP champs. (she tends to abuse her range to allow bursters to grab kills, Flash+ult saves lives and grants kills)

Shaco is a very unusual melee champ since he has much lower defense and HP than most other close combat choices - therefore he relies on surprise and map awareness. (his ultimate has many interesting uses - this adds ALOT to the learning curve)

Nidalee cannot win fights toe to toe without the aid of her trap, and even then it's a tough fight. She must soften up a foe before finishing them off with her chasing abilities. (IMO she fills the role of jungle/support better than carry)

(Another champ not mentioned above, Tristana. I cannot remember seeing a good one, but her jump ability needs to be used well, and likely along with flash to ensure a kill - bringing that jump ability back online for the getaway.... not an easy champ to master IMO, not to mention her ult should never be used to get a long ranged kill unless you are CERTAIN they will die, or else prepare for a lot of flaming)