Akali is nice champion benefiting from hard bursts to kill her enemies in a really litlle time. This is my build i hope u will enjoy it everytime i play Akali i use this and i personally think its good.

About runes ill just tell its very important to have both passive bonuses just from start so runes and masteries are done that way to give you enough of both AP and AD. Plus some magic penetration to kill them quickly.

Ill start building boots on lvl 1 cuz i have both bonuses from runes and masteries, of course i take 3 potions to help me survive.
Then build hextech revolver to spellvamp enemies.
Continue by finishing Sorcerers Shoes (there is also alternative of Mercury Treads when there is a hard AP dmg line against you).
Now start finishing Hextech Gunblade, and building Guinsoos Rageblade
As soon as possible focus at highering ur health, there is and awesome item for it Rylais Crystal Scepter.
Then choose Rabadon.
The last item has a lot of possibilities.
Dmg: Lich Bane
Pene: Void Staff
Or sustain: Guardian Angel
but i reccomend Lich Bane or Void Staff.

First max your Q for max dmg, then max E to burst Q.
Take R on 6 11 and 16 lvl of course.

Flash to easily escape or to easily get right to fight u need.
Ignite to kill or disrupt healing on targets like Swain etc.

Pros
Hard burst
Difficult to kill in stealth
Lovely girl, and she is sexy
Cons
ORACLE cripples her totally
Stun and u are almost dead
Needs her own Rune page and a lot of skill

Always let tanks go before you.
Wait until their carry shows himself/herself and then attack from behind.
Playing on mid is good against characters with skillshot abilities, playing top is not bad, but i dont like it.