specifically talking about champs that are bad ass when you master them....

orianna (op as hell when you learn how to use her)
zilean (can be a nuking, kiling ,machine, not just a support)
janna (another nuking, killing machine, not just a support)
yorick (dominating when you learn how to use him but takes time to really understand him)
viktor (three shot people with Q E R, not many people understand this)

Viktor, Cassiopeia, Draven, Anivia, Karma

He said they have to be ranged and not junglers.
Well, here's my list:

Cassiopeia. She's a tough one. Requires knowledge of her range.
Ezreal. He will show you a true display of skill.
Xerath. Requires probably the most positioning of any champion.
Viktor. Also requires a lot of positioning.
Anivia. Requires a bit of timing, positioning, and knowing your limits. Also very blue dependent.

ezreal is the hardest cuz not only do u have to shoot ur stupid **** u dont do any damage or have any range

I haven't played enough champions to properly pick 5. But I sort of think it's silly it took until page 4 for one person to mention Ahri and she hasn't been mentioned since.

All of Ahri's skills are effectively skillshots (2 don't require you to physically aim them but require good positioning or they're useless). She has high damage potential, but no real burst unless you land one of the game's hardest to use taunts. She is very very very squishy so you have to be extremely map and position aware.

Ahri is noted for her mobility, but much of the strength of her mobility comes in using skills on the run. She can still be a very easy champion to gank for many gap closers unless she has both her Ult AND Flash up.

Is she necessarily the hardest? Well, according to the difficulty meter she wouldn't be considered the single hardest, but she'd be up there. Clearly, according to the lists of people in this thread she definitely wouldn't be THE hardest. But I think a lot of people are overlooking a champ who should get a top 5 mention is a number of these lists.

Lee Sin. I'm talking JUNGLE Lee Sin.
Solo top Lee Sin not so much. The reason I say this is because Jungle Lee Sin relies SOOO heavily on that initial kick. If Resonating Strike doesn't hit, the gank is pretty much a fail unless your allies initiated and you can Safeguard Jump to them for the slow from Cripple. Also, Lee Sin isn't a mage. He's an AD. You're damage comes from the Tri-Force. In other words, it's "Ability, hit" six times, due to the fact that his three skills have a secondary ability within themselves. Also, remember that his passive gives you 40% atk speed on your next TWO hits. So, it becomes "Ability, hit, hit" repeat. As Lee Sin, you have to decide which you're going to do. Wait for the extra attack speed on the second hit, or go for the Tri-Force Proc? Also.. THE HARDEST THING ABOUT LEE SIN: You should never have this problem, but people always do. Watch when you ****ing use your ult. Too many times will Noob Sins try to get the last hit with the ult and end up Dragon Kicking people away to safety. Why use flash when you can just have Lee Sin kick you to safety...

Janna:
Support wise, she's extremely hard. You're spell order will not always be the same. How long do you channel her Q hurricane? Do you go shield first or go for the knockups and slows? When do you use the slow knowing it removes your passive speed boost? Just like Lee Sin, when is a good time to ult? To save your team? To push an enemy into your team? What if someone is coming from your jungle and you fail to see them? You might have just used your ult to push an enemy away when you could've just used your Q, slow, and waited for your incoming ally to secure the kill? I've also seen too many Janna's use her W and take kills from the carry.

LeBlanc, simply because of her late game. If you don't get the kills and last hits, you lose.
Anivia, lowest base hp in the game, but that's because of her passive.

Ezreal: This one is an iffy. His scaling is perfect. He's not OP or UP and everything about him works fine. If you think Ezreal is hard, you probably think landing a Lee Sin kick is hard.

Let me rephrase that last sentence...
If you think Ezreal is hard, you probably need to google "League of Legends, Smart Casting".

lol i was second person to post in this like forever ago