ps. i hate havoc because it just doesn't help much

1000 damage will give you an extra 20 damage on someone... and for 3 mastery points, it is a waste.

Bouboupiste knows what he's talking about.

Akali, like any other melee assassin, has very little team fight presence. You will be too squishy to do anything after you dive in. Your sole goal in a teamfight is to nuke a carry. But you also need to get out alive.

Moar resistances!!!

here is what I build: sorc boots and hextech revolver. don't finish the gunblade unless you have nothing else to buy!! it's horribly inefficient and it gives a lot of stats that you don't use as effectively as you could other items. I find akali's damage is sooo good after just those items just because of her cooldowns.

Next ask yourself this: Are you winning your lane? Yes? Get Rylai's. No? get Haunting Guise. Early health is the easiest way to build balanced survivability and Haunting guise is really gold efficient.

Both are great items. The slow is so good on akali, and makes her really sticky. Haunting guise is really strong early, it will give you about 50 mpen, which will make you deal true damage to most carries. DON'T FINISH THE LIANDRY'S!!! it is another item that is really cost inefficient until you get to late game, and it doesn't fit with akali's role anyway's. Go for the carries.

If you bought rylai's then your next item is the seeker's armguard, then voidstaff. make sure to farm that thing up asap... next you'll want a null magic mantle and to finish the zhonya's hourglass.

finally comes rabadon's and merc treads

Obviously, you can move those defensive items around in purchase order because every game will be different.

final build: merc treads, rylai's, void staff, rabadon's OR DFG, zhonya's, gunblade.

this gives you a great mix of damage and survivability for late game team fights. also i don't like lich bane because sheen is not great early because you waste the mana

Additionally: i would change up your masteries. 3 in Sorcery 1 in Summoner's Wrath . Use Ghost instead of flash. much better jukes. 3 points in Deadliness instead of Havoc (AD is much more helpful for the spellvamp), and take 2 pts out of Mental Force and put it in Brute Force instead. 1 AD is worth 2 AP, and end game 4 ap is nothing.

Hope it helps

Hextech is core, and rabadon's is very good, and so are sorcerer'sboots. I'm not a fan of akali but i would go hextech then rabadon/DFG (DFG can do Lich bane's job if you wat to burst someone, and you won't have delayed combo in a teamfight) depending of what your team needs (and your advantage/disadvantagee)
I think zhonya's, while not core, is still very good, and you should take Void staff as your 4th item or even before.

And during the early game, if you dominate your lane, don't disregard haunting guise. Early on MPEN is better than AP.
Don't disregard mercury's treads in case the enney team have zone CC (if they keep you cc'd for more than 1 second you're dead)

As a exemple of the need for MPEN : I won countless games vs akali with squishy mages (I won't talk about swain) simply because I had a unholy grail and the akali wasn't able to burst me OR took more damage when trying to fight. It was only 40 MR, it made me win my lane.

More on the Lich bane: You play akali as a glasscanon assassin. You have to all-in, destroy you target and if possible escape or at least give your team the upper hand. As you are akali, wich is not what i'll all tanky, you will need to burst and leave in most cases (if the ennemy is not neglecting pinks and oracles)
So either you have 80 AP and a 50+75% ap damage proc (you won't proc twice when trying to kill someone fast) OR a 15% max health nuke + 20% damage amplification. If you play as an assassin, go DFG
(Fizz is an exception, and that's because he procs lich bane with Q and have his E to survive and proc it again)

TL;DR On a game with the ennemy adc with a decent level of farm i'll go Hextech rabadon's DFG (void staff if there is already a Bulkwark)(I love rabadon's, not sure it would be good) and zhonya's if i have the money (to stop them from focusing you, and have CD back to try to kill another guy)

Don't forget the MR aura fron Bulkwark, often unseen, always lowering your damage

PS: try to get your guide reviewed by more experimented users than I, they might give you better advice and I might get akali's build wrong :)

Well, first of all I don't think there can be many items crossed out of my core items list. Hextech is THE must buy item for akali and so is the sorcerer's and I hope you agree. About the rabadons... well when I think much more of buying when I play other players but it is not bad with akali actually. And now about the lich bane, okay, in fast paced teamfights I can say you have to do everything SO fast and my not get so many from lich bane, but if you do a delayed combo (not for so much) you can get your lich bane proc plus lower cooldowns which means more combo.

And now we get on the MR and the void staff... First of all the items are not EXACTLY with the order I purchase them. As you can see the second one is the enchantment and most of the times I may not even buy one. The truth is however MPen is not the first thing I go for when I am akali and still I dont think it's the most needed thing.... Maybe I lack of experience, but so far I have been playing akali somewhat like this and I don't think it has been less than excellent....
But I'm curious.... what would you buy for akali? I mean what would you suggest if I don't take what I take?

PS: Really thank you for all your good will and advice and I will try out what you told me

What I love about your core build is the fact you forgot you won't play against 0 MR items ennemies.
You spend 3300+3000+3400+1100 (rabadon's lich bane hextech sorcerer's boots) for a total of 10800 gold and you haven't any way to survive if you're caught. And your full build with a Void Staff as a last item... It should be bought much earlier. Even as akali, you won't always have 16470g before needing MPen.
TL;DR: On any assassin penetration is not optionnal it's a must-buy.

PS: Ask yourself if the lich bane proc you'll get (and your combo let you use only one) are necessary and if other items won't give you more bang for your bucks.
PS2: Get your grammar and spelling reviewed, and read jhoijhoi's guide to making a guide here: http://www.mobafire.com/league-of-legends/build/making-a-guide-101506