Hey people, so this is my first build so try not to be too judgemental, but I've been playing straight Akali for a long time now and I feel I have a pretty good handle on her as a champion. I've looked at a bunch of other builds in the community and I've noticed some things that I have different in how I play her so I decided to give it a shot. Lemme know what you think in the comments section, vote on how you think I did, and I'll try to make updates as needed.
Thanks! :)

10/2/2011 Update
Besides the patch that tweaked Akali's Shadow Dance so she attempts to attack her target immediately after Shadow Dancing to it, there really hasn't been any changes, so this build remains as valid as it always has.
Changing and tweaking some sections but that's about it.

1/22/2012
Since the mastery tree has been changed and Akali has received a minor nerf, I made a few changes to the build. So try it out and please leave comments! :)

Pros
Unmatched Burst Damage
Incredible Chasing Abilities
Fun to play
Great zoning/experience deny ability
Stealth/Intimidation
Good Ganker
Good Harass
Great Damage Output
Good at escaping


Cons
Squishy!
No Crowd Control (Twilight Shroud Slows but not much at all)
Steep Learning Curve
Escape/Defense can be sharply countered by Oracle's Elixir
Skill Required to Play well

-Quintessences and Glyphs of potency to give that extra punch early game and to help activate her passive ASAP.
-The hp per level seals help considerably with survivability to take a few hits while dropping someone especially mid/late game.
-Magic Penetration Marks to combat Magic Resist and to make the Ability Power runes really count.

I run a straight Offensive mastery tree because personally I know how to play Akali carefully, and if you're smart, you won't need to put points in defense, and your dmg output will be SILLY.
-Improved offensive summoner spells is great for Akali because since this is a hybrid build, it just gives her more attack strength.
-The extra Attack Damage and Armor Penetration helps Akali really bite into enemies throughout the game.
-As far as defense goes, the bit of extra defense never hurts for those who are just starting out as Akali, or just don't feel safe playing a glass cannon build.

Skill Sequence is pretty straight forward, get up your Q (Mark of the Assassin), every chance you get, at level 2 get your W (Twilight Shroud). Laning is so convenient at this point because your opponents are afraid to approach the shroud lest you unleash a Q onto them and then disappear back into the shroud before they can retaliate.
At level 6 get Shadow Dance, the ult, of course. From there just follow the sequence.

Q --> R --> Auto Attack --> E --> W (to hide)--> (Repeat) Q,R,AA,E

If you can pull off that skill sequence as fast as possible. Enemy champs will barely have enough time to run away, let alone retaliate.

-Although people frequently argue about Long Sword versus Doran's Blade to start, I feel that Doran's is the way to go. The extra health helps lane longer, the +10 Damage immediately activates half of her passive, and the little bit of Lifesteal helps with laning as well.
-From there I get Boots of speed to help with mobility and chasing/escaping early game, then upgrade to Sorcerer's Shoes. The reason being at that point in the game, with AP runes and a few points in Mark of the Assassin, a little bit of Magic Penetration goes a long way, and something like an Amplifying Tome is relatively unnecessary.
-Rush Sheen immediately after getting the Sorcerer's Shoes. Not many Akali players get Sheen as the 1st item after boots, but the added early game burst is a must-have. Sheen helps greatly with early burst damage and those who don't know Akali's potential this early game will pay dearly.
If you don't have enough to buy the full Sorcerer's Shoes the 1st time back to base, get Boots of Speed and just take an Amplifying Tome as well to start building your Sheen.
-Several Akali players will then get a Hextech Revolver because of the added spellvamp and ability power. I however favor building the Bilgewater Cutlass for a few reasons. First off, the Vampiric Scepter as part of the recipe gives you great lifesteal, and not to mention once you get the pickaxe as well, her passive gives you even more healing capabilities. If that wasn't enough to convince you, Bilgewater's slow and damage active is incredibly helpful to Akali early game because she doesn't have much in terms of crowd control.
-After the Bilgewater, then build your Hextech Revolver and complete the two for added range for the active, double the damage, and other improved stats.
-Hopefully you've made it to this point in the game without dying too much, because before now Akali is relatively squishy and a Giant's Belt is a great added hp bonus. Now you can take a hit or two before needing to throw down the shroud.
-Once you have the Giant's Belt, build the rest of Rylai's Crystal Scepter for a nice slow passive effect and more Ability Power. Remember to always grab the more expensive item (Blasting Wand over Amplifying Tome) if possible because that'll be your last item slot until you complete Rylai's.
-Now that you have an active slow, slow on spell hit, and decent Ability Power, enemies are going to have a hard time escaping your wrath. For even more punch, grab a Rabadon's Deathcap next, with the Needlessly Large Rod first if you have the money.
-If the game isn't over by then, get Lich Bane for unbelievable burst damage, plus some nice Magic Resist and extra movement speed.
-If the game STILL isn't over, or you're just incredibly fed at this point, trade in your Doran's Blade for a Void Staff if they're stacking Magic Resist, or another Rabadon's Deathcap for even more facerolling action.

Flash - For Akali I believe that this is better than ghost. The reason being if you drop your Twilight Shroud near a wall you can walk over to the wall and flash over it while your enemies are waiting for the shroud to disappear. Not to mention it's great for ganks and for that last hit after landing a Q on your enemy if you're out of Shadow Essence Charges.

Ignite - Early game for Akali this is incredible. It finishes off people who are retreating to their tower after taking a beating, and most importantly, if improved offensive summoner skills is being used on the mastery tree it'll give her a nice bonus AD and AP bonus.

Last-Hitting is CRUCIAL.
Monitoring all of the enemy minions at while being aware of your enemies isn't easy but it will certainly pay off in the long run. Mark of the Assassin (Q) is perfect for last hitting because of it's range if you're being pushed away from minions due to ranged enemies etc.

Twilight Shroud (W) is incredibly helpful while farming for a few reasons.
-Harassing enemy champions becomes wonderfully easy at level 2 because if you drop the shroud behind/in the midst of their minions, you can get close to them, exp deny, and get a double mark explosion combo on them if they're not careful.
-Using the Shroud as an escape mechanic is also incredibly helpful if you're being chased to help make it back to your own turret or juke an enemy in the midst of the jungle.

Basically just harass the enemy with Q when you can, if they get to close, punish them with a Q or a "double mark combo" which is much easier to pull off once you have your ultimate. This entails throwing a Q on the enemy, and then waiting a few seconds for your Q to cool down but not so long that the Mark expires on your opponent. Ult in, explode the first mark, and by this time your Q will be ready so toss that on them and explode that mark as well. This will crank out great burst damage that many players underestimate, giving a fantastic advantage. If you're taken by surprise with a gank or if they have heal and try to 1v1 you, toss down your shroud and wait till your Q cools down and jump out to finish them off.

Some notes to keep in mind.
- Shadow Dance (Ult) can be used to escape as well as chase by using it on other enemies, minions, or neutral monsters.
-While around enemy towers, while in the shroud, you are not stealthed.
-If you're engaging a fight and you've already bursted a Q and you're waiting for the cooldown, don't forget about your Crescent Blades (E).
-Always make sure you have enough Essences of Shadow (Ult Charges) to complete what you want and/or ensure your getaway.
-Always keep an eye out for people with an Oracle's Elixir, this makes them able to see you even while stealthed, and the fact that they spend the gold to buy one means they think you're a viable threat and thus will be basically guaranteed to be focusing you so be very careful, and ask your team mates to target that person first, because the sooner they die, the sooner you can go back to punching them in the face.

I know I'm not the best coder or anything and I'll be working on that but I just wanted to put this out there to see what people thought.

Good luck if you try this out and have fun embarrassing people! ;)

Here's a recent streak I've been on! :D
By dpictor