I have been playing Akali for ever now and i personally think shes OP even though shes my main. If in the right hands she can turn a gank into a double kill , turn around a losing team fights into a win and carry games single handedly.

This guide will show you how to accomplish this , now im not saying if you read this u will become god but you will get close.

Now i do understand you could put Akali top, however i find middle alot easier and easier to get farmed up on which is needed. The other Ap carries in mid like Malzahar,Brand etc. will not one be able to out last ur sustain and two be able to out poke u with you Q and dash.
Play Video
Explains alot of things if u want a visiual

Pros:

Amazing damage dealer
Invis. cloud
Can stick to people very well
Once level 6 gareenteed lane dominance
Will melt AD carries
Easy to farm with Cresent Slash
Fast snowball
Extremely Versatile
Shadow dance makes for great Jukes


Cons:

Can be focused with cc
Oracle/vision wards makes her cloud useless
3 dashes
Ignite reduces her Spell vamp

I go 21/0/9 because of the good Magic pen,Improve Ignite,the 3 attack damage which is needed to proc your Ad passive at lvl 1 , and also the armor pen and damage inc.

The 9 in utility i like mainly because of the xp increase but if u dont mind u could go 21/6/3 and pick up Resistance and Hardiness with Good hands.

Glphy of Greater Strenght is needed to activate your attack damage passive at level 1 along side Brute Force which is in the masteries you will reach just over 10 attack damage which will give you around 10% spell vamp.

2 Greater Mark of the Insight -Its the next best thing pretty much the magic pen will allow you to cause more damagee by avoiding the magic resist.

9 Greater Glphy of Potency - It gives you an great early game with and improves your damage also helps with your passive proc. However u could take AP per level if you feel most games for you last long, like everything else its personal preference.

9 Greater Seals of Vitality - This helps your late game primarly, also allows you to stay in your game as it acts like a catalyst.

3 Greater Quintessense of Potency - This is where all your early game damage comes from and this is what procs your passive which gives you a great damage inc. THESE QUINTESSENSES ARE A MUST.

I like to run flash and ignite , flash incase ur stuck in a sticky sutiation in ur bubble or just need to get out, also on akali you can use it as a gap closer if ur ult is out of range.

Ignite for that last bit of a damage and this also helps you destroy supports in which there will always be one in Ranked games.

Ghost is okay but i would still pick flash and possiably Teleport but everything else is just too weak to be used on Akali.

Maxing Mark of the Assassin is no doubt the best thing, it causes your auto attack after it has been thrown to deal bonus damage and its a great poking abaility with a very short CD.

Next i like to max Twilight Shroud over Cresent Slash , now this is an personal choice however since your going mid it is the best thing since A. It gives good magic resist to avoid some of that AP damage B. An invis. which plays with the opponets mind C. It causes easy escapes just incase someone comes for a gank.

Cresent slash i max last , its a great farming abaility however i feel its a energy dump abaility in which means that use it if everything else is on CD.

Shadow dance, Now this is what makes Akali an assassin and an OP mid , Allowing you to dash 3 times is such a big deal as it allows you to stick to anyone you wish. In lane use this in combonation with your Q to cause insane damage simply throw your Q wait for the cd to comeback then dash Throw another Q proc that and then walk away or dash back to your minion.

Since you will have your two passives active at level 1 you should start with boots and 3 health pots as it will help you dodge skill shots (in which most AP carries have) and the 3 health pots will give you good sustain.

Hextech revolver as your first item is a must, it provides Amazing lane sustain and more abaility power to add to your damage, this item makes you win almost all damage trades

Turning your revolver into your most important item Hextech Gunblade, this item has every single thing akali needs , from Ap to spell vamp to attack damage and even a slow which allows you to stick to your target .

Next item is Rylais, This will give you survivabilty from the hp you gain and a slow in which makes sure your opponents cannot run.

Guardian Angle is also a must, this items makes the other team have to choose A. Blow all there cooldowns on you in the hopes to take you out of the fight for 3 seconds and then have to rekill you or B. Avoid you , in which case you go berzerk and kill there whole team.The armor and magic resist is also a huge plus

Lich bane, Just another insane damage increase with the abaility power and the sheen causes ur attack damage to deal over 500 damage which is insane considiring most Ad carries dont go over 2k

The next item is something you must choose depending on the other team, Banshees veil if your being focused and they have a good amount of AP damage, Deathcap if your just owning and feel that you need more damage to kill their ad carries and last a void staff if they have alot of MR.

Just wanted to add this , Akali is one of the easier people to farm with in the game , you have alot of tools , since ur based of energy and ur pretty much only limited by cooldown dont be afraid to spam , you can last hit with your Q or proc it to last hit and cresent slash is also good to clear large waves.
At the start always priotize last hitting over harresment since your 1-5 is pretty weak.

In ranked play or even normals make sure you allow your tank to engage and take the frontal burst before u enter the fight , one stun outside of your bubble you can be melted fast , once the fight your focus is their AD carry or AP carry, hop on them as soon as possiable and DO NOT stop until they are dead. That is the point of an assassian! TO kill their Carries!