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...
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".