This is my first guide/build, so please be nice!:D

Akali is an excellent assassin to pick out easy targets such as Ashe, Corki, Kog'maw and other squishy champions thats important for their team. Lets get to the runes, shall we?

For runes i Choose to go for:
MPen red (u can go armor pen if u like to)
Armor yellow
And flat AP blue and quint (u can choose to go ap per lvl if u want)

''Why not attkdmg red, so u can activate passive both passive without item''?
Because it doesnt really matter since long sword is the first item to get, u will still have both passive at lvl 1

''Why armor and not something else for yellow?''
Actually, yellow doesnt really matter. It just looks good to have some extra armor!;)

And the AP blue and quint is to activate the extra m dmg at lvl 1, it looks good!;)

Early:
ALWAYS try to go mid. Get a longsword and pot and u will have that small amount of spellvamp from passive wich will make a difference in laning phase. Play defensive and just keep on last hitting with Q and try to get as much gold as u can until u reach lvl 6, that's when the fun begins!

Mid Game:
Get a sword of occult and some boots and ur ready to gank around.

If u managed to get a few kills u should go back and get ur spellvamp thingy wich is really good. Combined with ur passive. Your pdmg will deal alot of extra magic dmg!:D
Now u should just have ur eyes on their squishys and kill em whenever u can! But dont get 2 greedy, remember that u got a stack to keep.

Lategame:
Now u should have rylais and around around 15 stacks if ur doing well. Just follow the build and keep on owning the squish ppl!

Flash is a must. It will save your *** sooooo many times. Don't play akali without flash!!
As for the 2nd summoner spell i usually choose to with exhaust. But ignite works well to, or maybe ghost. Depends on what u like to use and what ur team got/lack.

Never really tried her with anything else then those four, but im pretty sure there is no better spells for akali.

I use Q and wait for it to mark them first before u go in with R. Alot of akalis rushes in with R as soon as they hit throw Q. THAT'S WRONG! Q is a kinda slow and it takes time for it to hit the enemy. Ur R is way faster and u will auto hit the target before the Q has even landed. That will force u to hit the enemy twice to activate the mark.

Then u use ur W and wait for ur Q to get of cd. When its off do the same as above. Throw it, wait a sec and then BAAAM R! Remember that Akali is a burst dmg dealer and shouldnt be in the fight all the time since she is really easy focused if ur doin well.

I rarely use my E but its usefull sometimes for the extra dmg or if u need some health back and u use it in the middle of a minion wave.

Pros
AMAZING burst dmg!
Hard to catch
Deals both massive Pdmg and Mdmg
Really fun to play
She's a flippin ninja!

Cons
She gets noticed really easy and targeted often
U need a good tank to do a huge impact in the teamfights
A bit difficult to understand when u should strike and when u shouldn't
Boring Taunt/Joke

This was it. Rate and comment :) Feedback is always welcomed and remember this is my First build/guide so it might not look so good 0_o