I Have decided to keep this guide pretty short as i am sure the majority of you know how to play akali. Please comment and rate after trying this build

The reason i have put my masteries the way i have is because akali is a burst damage champ. Taking 21-0-9 gives her the ability to jump in, hit, retreat and regain health quickly. putting a point in offense on Exhaust if you choose to take it.

Firstly, rune build should activate one of her passive aspects from the start, usually her spellvamp. This means flat AD marks. Seals you should choose whatever you like, and glyphs - i prefer AP per level. 9 of those + 2 similar quints give me 41 AP at 18 champ lvl, and combined with rabbadon it is even more. The way i have done the runes here benefits akali the most with her passive.

When game starts, buy the item that would activate her second passive aspect. If you have AD runes (and thus spellvamp active) you should buy tome of AP and health pot.
Your items in the end should be something like:
Mercury/sorcerer boots (depending on number and strength of enemy's CC), Hextech gunblade, Trinity force, (get Sheen first early game) Rabaddon deathcap. Other items are situational. I'd say buy another 2 gunblades for awesome spellvamp, but i heard that riot made some changes that accidentally influenced spellvamp on gunblade, forcing it to not stack. You don't need Rylai's, because your gunblade slowing nuke, Trinity % of slow and W ability will slow enemy enough. Not to mention Exhaust if you would take it.
One of the possibilities is to build Mejai's and Occult if game goes very smooth.
Or some defensive items, if enemy excels at one type of damage. Example - enemy Xin would be frustruated with Randuin omen, Yi - Thornmail, Malzahar - Quicksilver sash, any mage - Force of Nature, any stunner - Banshee's veil.
Skill order build is Q>E>W (those three in whatever order, but obligatorily 1 ability point in each ASAP) then give preference to Q and E equally. E is good for farming minions, Q for harassing. W is good too, but it's scaling only gives some protection, while most of the time you just want to be unseen by the enemy instead. And choose R as soon as it is possible, of course.
Skill order use is Q, then wait for it's cd until it is almost ready, then R, autoattack, W (place it so that enemy would have difficulty escaping), Q+autoattack, and only then E. If enemy is still alive, i suggest you usually flee, because Akali is good burst type but is weak when all ulti essenses are depleted and energy is low. Now why i mentioned Q cd combo? It's just a matter of better burst, so you can do additional Q-hit damage, then repeat it again because it is refreshed. For this reason, don't ever believe people who say that blue buff and blue elixir are bad for Akali because she has no mana. CDR is a good addition on her, though its not that important as AD or AP.
Good summoner spells on her are Exhaust, Flash, Ignite (combined with corresponding mastery that gives 10 AP), Ghost and may be Heal in early game. She's not that good jungler early game so i think you shouldn't get Smite. Other spells on her are too much of a waste to even talk about them.

You may prefer Rylias over Gunblade, because you benefit more of the Hp so you aren't so squishy (Hp per lvl Seals also a good choice). So you can survive in mid game. Also you have a permanent Slow which will help you even in Late Game, without an 60 sec CD!
The Damage of Gunblade is a good choice in early/mid but gets kinda useless in late game imo.

I Have taken Flash and ignite. The two spells are the best on Akali as she does burst damage. Allowing the ignite to finish them off and the flash to get away quickly. Another good summoner spell is exhaust.