Akali is in my opinion the best burster ingame, and becouse off that best to get high and feeded asap (Best carry). I am not gonna go all out with text in this build/guide just gonna go through the cons and pros and how to show the true power of Akali.

AD in Marks is a must to proc the Passive Spell Vamp of akali.

The HP-regen runes is becouse she is a surviver and when going mid u will harass the **** out off em with your mark & invis - and just keep your health up during until lvl 6 when u jumps and kill who ever the f u meet.

AP in Quint & Glyph is becouse well Passive & DMG.

Specing DMG & Survival since that is what Akali needs to become the hunter she is.

First off i start with Dorans Shield - Since it got the HPregen, life & armor that she needs to stay alive and to hold mid without any probs what so ever. I stay out midlane until i got either 1230ish or 1550ish (Mejal and Shoes or just mejal). What u need after Mejal is HEALTH there by the Giant Bealth, lots of peeps go with more AP but i say that staying alive with Mejal is more important. Build up the Rylals Scepter and after that go with Rabandans Deathcap and depending on your opponents build a defensive item at last (I ussually go with Gaurdian Angel), this is a pretty expensive build but i will show you how to complete it in every round!

Your mark is the foundation off your DMG & can be proced x2 under a sec - Meaning that you can first throw and mark the target, wait just a sec to get rid off the CD then jump him with Ultimate - MIDAIR throw again & Throw out your invis on the target! DONT HIT EM WITH AUTOATTACKS!! Stay invisible until u get your mark without CD - Throw your mark again then jump em directly with ulti - and do this all over again and without wards/oracles no one can beat u when full health (Best example i got is when Veigar went from bot to mid to gank me when i was at 200 health (just dinged lvl 6 and nuked fiddle mid) i nuked him without any problems when he was full health! JUST DONT DIE WITH AKALI SHE IS A SURVIVER

Flash - Becouse before lvl 6 akali is kinda waste without flash in mid - this can work as your ulti to proc your mark, and just an overall awesome survival skill when combined with your shroud!

Ignite - Chose to go with ignite since alot off champs got away earlygame with less then 100health, and also becouse u dont know if you are going against fiddle and i promise u without ignite he is a real pain!

When u play mid just play like the underdog, the one u go against must think he got the upper hand in this case u WILL get to rape that champ very easy. And remember to not go home early on even at low health - that is were the HPREGEN comes in & combined once again with the shroud makes you a unkillable target early on - atleast when u get this to perfection like i, played akali for over 500games!
When u have killed your opponent in midlane - fakeportal back home and run bot/top and grab yourself a dblkill before returning home and buying mejal&shoes - go back kill your midlane again then run top/bot again - and just keep on doing this and you will win for your team!

This build works just as good in ranked - didnt thought it would but had no problems what so ever since this build with Akali is pure awesomeness! And if they ward mid - u do the same and just destroy that ward (they wont buy and buy and buy new ones so it is no problems everyone thinks akali is a gamble ranked but srlsy no she is not...

Pros:
Very high DMG
Very good survivale
Underestimated since noobs cant play her

Cons:
Prio 1 Focuse in teamfights
Early on ganktarget

But when u know these cons u also know how to expect and avoid it!

Akali is a awesome ganking partner - specially when going mid since she can move there very quickly and get a dbl kill, in teamfights Akali should solofocuse the highest AP user since she can nuke down that target just as fast as the other 4 can nuke the second! Akali should not be seen before she jumps her target since she will most definitly be targeted and nuked very fast so keep in bushes/forest and wait for your opportunity becouse as i have written before she is a surviver!