Akali is technically easy. What is hard about her is that she is judgement hard. This means knowing what it means to play passive-aggressive and anticipating what your opponent will do and not baiting yourself into thinking you had the lane easy once you hit 6.
Might seem like a joke to go mid with him, but seriously if there ever was an Akali counter...this would be it. Want vision? Np. She's getting away? Oh look, I had a q on you. Oh, and let's just make it easy for my team to kill you with ult. Better hope you can jump away and heal fast enough. Seriously though, he's usually too bulky to take down 1v1 and the vision hurts a lot. Plus he's better pre-6 than you unless he sucks.
Even if you're good, going against Lee sin is a waste of time. You can't roam if he's mid or top with you because until you build a stock of ap, he'll shred the tower faster. Like you, he's not afraid to tower dive either but you don't have as much freedom to tower dive him in return because he can just fly to a ward and let the tower have you.
Annie is also hard against her pre-6. Still a bit hard because of Tibbers and stun and burst but Akali is a general soft counter over squishies.
And no, Kat doesn't counter Akali because Kat's ult is too exploitable. Diana is harder because of the shield but if you play smart she's tolerable. I'd rate Kat 5/10 for "skill dependent" and Diana 6/10 if you're going in as Akali.
I never had a problem with Fizz so long as I intend my R's to execute rather than try to 100 - 0.
A general rule for Akali is not to think too much of being harassed pre-6 as long as you can at least get a revolver by level 6 and to keep in mind that you're not the only one who can down your lane opponent after 30% hp is gone.