3x MS Quints (You can run AP Quints here; MS Quints is a personal choice of mine that I run on everyone)
9x AD Red (Most people go 7 AD/2 MPen but I find that the differences are negiligible either way... 1.9 MPen or 1.9 damage + some spell vamp? Up to you)
9x Armor Yellow (still shopping)
9x AP Blue (Scaling since the moment you hit 6, these are equal to flat and then better and Akali REALLY shouldn't be doing anything pre6)
Masteries are 21/9/0 (Essentially, all AP masteries other than Mental Force, which I swap out for Brute Force and Summoner's Wrath; Also, defensive masteries are Durability 4/4, Veterans Scar 1/1, And then Hardiness/Resistance 3/3, Resistance/Hardiness 1/3 depending on expected lineups)
With these masteries, you can trigger her passive early game, allowing spell vamp sustain. Also, since the rest is AP, your mid-late game will be stronger in general.
I typically rush Hextech Revolver and then Rylai's.
The sustain of Hextech and the health of Rylai's means that Akali can't get bursted down; also, the increased AP is always good and the slow from Rylai's is a great chase mechanism in case your R is down for whatever reason.
I can go Zhonya's or Abyssal Scepter is I'm getting wrecked by a specific type of damage though.
Then, Sorc Shoes.
Then, depending on the game, I change my build. If the game is going really well, I grab a death cap (and then Lich Bane typically). If I need more sustain and/or more chase power, I grab Hextech Gunblade. If I need a little more chase and more damage, I go lich Bane.
Eventually, my ideal 6 inventory item assuming no survival issues would be Sorc Shoes, Hextech GunBlade, Deathcap, Lich Bane, Void Staff, Rylai's.