Akali main here.
Akali is very weak in team fights, therfore, grouping and team fighting is a major counter to her, and its a reason why she is so weak in late game.
Akali also has a mediocre laning phase, meaning, she will lose to strong lane bullies in lane.
Example of counters in mid lane: 
Example of counters in top lane: 
Also, good supports are very difficult to play against: 
Please don't underestimate her early game. Akali's early game is not the strongest, but its not weak either.
She can easily win and snowball a lot of match-ups even pre level 6. Once more, do not underestimate her.
edit: saw someone mention quinn.
Please, do yourself a favor and do not fucking play adcs into assassins, unless you plan to lose the game ;)
LB is one of the most hard counters in mid lane btw.
So, your choice against Akali is the next one:
Counter pick her.
or
Pick a team fight pick, and respect her damage potential. Which means you counterplay her with team play.
or
Pick a strong lane bully. There's many lane bullies you can pick blind and do well in Akali.
or
Win the skill match-up which is what most Akali match-ups are. And this just means you counterplay her by being the better player.
As she has no skill shots, she relies on dodging her enemy's skill shots in order to win fights and trades. So, if you hit those, you win ;)
You might think Akali is an easy and noob champion. But there's a lot of micro managing and skill shot dodging involved in order to play her properly.
You can still win with Zed, Talon, Rumble, Fiora, Oriana, Ahri, Syndra, Riven, etc against her.
But you will have to hit your skill shots and respect her damage output (aka: don't stay in lane with 50% hp for god's sake).
Akali power spikes at:
-lv 2 when she gets her W
- at lv 6 when she gets her ults
at
at 
at 
at 
and at 
oh and at 
(usually required for team fighting akali)
After this, her power starts to get lower. As you can see, almost all items except zhonias is an item obtain in the early and mid game, and useful for that time in the game.
While you call Akali the pinnacle of anti fun.
I have to remind you of her weaknesses:
- she's melee
- her main mobility can only be used aggressively on enemies, meaning, she has way into fights, but no way out.
- she is squishy
- she does mostly magic damage and a bit of ad.
- she has no wave clear (so, you can easily siege her turrets)
- she has no cc (so, once she falls behind, it become difficult for the akali player to do something, and doing something is a testament of their skill and game understanding)
Meanwhile, the likes of Cho:
Tanky as fuck, loads of CC, huge ass true damage burst. And all of that for just not having mobility...... yeh right. And Akali is the "unfun one".
ps2: ignore people that tell you to stomp akali pre6
if you follow their advice when you do not have a counter match-up or a lane bully, this will only get you killed and have an akali snowballing even pre lv6.
Do not underestimate Akali.
Her weakness is late game, not in the early game. Mid game she can 1v2 the enemy adc and support unless they are undertower, because that's when her power spike happens. This is true for all assassins btw, not just akali.
The problem is that in late game, adcs just 2 shot assassins, so, they have a harder time due to 'delayed' burst, even if they are alone.
and maybe you can leave base once every 3 mins lul
you'll be able to murder the fuck out of her, but if you instead have champions that wants to win by range or burst, then you're probably not gonna win against her.
with 
?
? ((how do I get the hm? sticker for league?)
, 
Out-1v1 her: 
, 
CC her to no end : 
, 
, 
, 
, 
Or out-tank her: 
- see previous, 
then ur lucky to choose a counter pick like 
, 
, 
. She needs a rework no doubt but until then beat her face in with these picks and then some.
