Most of the easiest champs will be the older one when designs were simple but think about this:

You wouldn't think a new champs would be so easy right? Well, look at all of her abilities and what she gets?
Passive is extra damage, right click on someone and after 5 attacks, you deal extra damage. Good job you. But that's not all, want more damage? Just build **damage **and your other abilities do more damage.
Q - Press Button, do damage. No aiming or anything, just press that button. If you have lots of AD **damage **it does more damage.
W - Be careful now, this is a skill shot. I know right, a skill shot you have to 'aim' but it's ok, it has a massive range and procs your passive too to get to extra **damage **quicker. Plus there is no warning that it's coming, unlike Jhin's long range attacks. Also, if you hit it, and have enough AP damage, you get cooldown refunded to do it again sooner to deal more damage.
E - Now this one is just movement speed but you get extra attack speed after, get enough attack speed and you become invisible so no one can attack you.
R - Press button, get shield and puts you right next to someone your passive has marked. What do you mean you are miles from them and didn't hit your W? That's okay, your passive can be proced by team makes CC so doesn't matter where you are, you don't even need to have skill to do damage to someone, Press R to be next to them, then right click and Press Q to deal damage.
Good job, you've mastered Kai'Sa.