~~~~wanna win? Wanna carry your team? Here's a build for you~~~~~~~~~~~~~~~
Gonna start off right away with just stating what lots of people don't realize. Ahri's ap conversion to skill damage is really low. Means you waste a lot of money stacking into ap when you will not see that great of a outcome in damage.only 33% of her ap converts to damage. This build is For dominion. I don't use this build in normal. Dominion and Normal, are like 2 different games. Builds should never be the same. I also take heal, just cause I know how to use it to my teams advantage. If you don't know how to use it. Use something else.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A normal game you should be able to complete most this build. That is why it makes it vary effective. (they changes something, I was only capable of building void at end of build like once every 5 games. even when I'm on fire I still only get to complete it with less than a minute left in the game now. so just be aware its hard to reach)
Sorry for any spelling or grammar. May not be the best at it. But u don't need to be to be good at games :)
btw. minus building archangels and mejas in normal. I use this build to and dominate with it. so most it still works in normal. ~~~
Nice thing about this build also. It's not like EVERY other build posted.~~!
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with my build:
100 magic def - 75 magic def = 25 = about 13% less damage. 400+ 100 = 500 - 13% = 435
if you count void staff. 40% magic pen. Heres the new math
100 magic def- 40 magic pen. = 60 - 77 magic def = -17, = about 10% more. 400 + 100 = 500 x 10% = 550. see the math? AROUND 165 more damage! if you want you could count the 10% magic penetration also from masteries.

Don't forget to factor in Scepter magic pen and void staff
with out my build:
100 magic def - (ill be nice and say 10, even know you plan on none) = 90 = 45% less damage. = 385
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Another update: You can pretty much build these items in any order, and they will be just as effective. I still start with magic pen boots. Order im going as of current is
boots, guise, codex, Scepter, Hextech, evil tome, ancients, Void staff- and constantly 3 mp pots on tap. """"""""""""""""""""""""""""""
if you like the build please +1 it :) ill love ya and + rep ya to :D
Update: another nerf on ahris ult >.>
Updates 2-28-12: still godly.

I use my Magic pen/armor runes with a crit rune. Crit rune is so you always have least SOME chance vs none to crit. or you can use magic pen runes and ap runes. Like i said. Ap don't pay off enough. I like to have that extra 10% damage reduction.
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Reason I go with this set up is for cooldowns/magic pen. This set up gives you the greatest Cooldown reduction. This build is aimed at magic pen/cooldowns. Use what ever you feels right. I feel this is best.

when shacos/twitch/eve are present in game. Buy orcals as long as ur not dieing and making waste of your money. You will get a lot of kills from them with orcal. Do to your W does a ton of damage to defenseless chrs.
Magic Pen. Boots: I start with magic pen boots right away.
Also start with 2 hp pots and 1 or 2 mp pots. Do to you burn through mp fast with her. If you play right. By the time you use your pot and need to return you should have enough money for your Hextech.
Hextech: If you want you can skip this for a item and Guise first. However its cheap and still useful. Just more useful later on. Item becomes great for casting your Q on mobs to drastically increase you life. Helps a Ton with Your W when kiting people. Your W is powerful and will heal a good amount of life with this.
Haunting Guise: Items one of the few Magic pen items. But also is vary cheap vary good to get for early game.
Abyssal Scepter: Again one of the few magic penetration items. However it also helps with your magic def that your chr lacks, and helps all your team.
Will Of The Ancients: Time to finish it. Now its more useful and Will cost you little.
Evil Tome: Time to get your Cd. Adds ap/cd and helps your mp that you burn up like no other. Got the right items? Your W will have a 3 sec cooldown. Witch will be hitting alot of damage at this point.
Last Item: Ok this is were you need to fill in yourself. I normally go with Void this late in game. However you don't always have people with high magic def. If you wish you can build deathcap. However this build with void will leave you just enough time to finish the game. You more than likely wont get a chance to finish deathcap. Same goes for rylais.
But your last item can be one of these. Void,Deathcap,Rylais
note- make sure you buy mp pots here and their. not hard to run out of mp with her, specially when your on a roll.
If you wish to use this build in Normal games. it is possible. I would just recommend working Archangels into the items. or you wont be able to stay in fights do to lack of mp.

Simple. Get all 3 skills right away.
Master Fox fire. Your strongest move.
Than Master your Orb. Aoe/ Also strong with true damage.
Master your Ult as soon as possible.
Master charm last. Its a nice useful move. But the flaw to it is- your not a tank, and it makes people come to you. Ahri cant afford to take alot of hits. it also doesn't do alot of damage and requires a high amount of ACC to hit. HOWEVER deadly combo's are used with this skill. I will share them in Summary to help.
update: Depending on how you play. I've lately been going back and forth on learning charm and orb. depending on who Im playing. I master fox fire as always. Than level 2 charm 2 orb 3 charm 3 orb 4 orb 5 orb than 4 charm than 5 charm. The extra 2 levels gives u like .5secs witch helps u get out of trouble.
Updated the skills at top to this ^

I use heal. I like it. always have. Weather its saving my ***. Baiting the other team. or Saving my teammates. Theres many uses for it. However not everyone can use it to its potential. I for one can say I've mastered the use of it. So unless your really skilled with timing on it. use something else.
Ghost is the best dominion skill. enough said.~~~~

Ok. Here's some info that you will want to know.
Ahri is a vary strong Chr with this build. However. Do to your high damage out put. It is vary easy to die. YOU WILL be highly targeted. No matter how good Im doing the other team always targets you. So you need to play smart. Use your W as much as possible when neer a champion. With this build u can Take out most chrs 1v1. HOWEVER some champs obliterate Ahri. Chrs with lots of disables. Lablanc has been the hardiest targets to me to kill with her. When she uses her bind it disables your ult and does not allow you to use it on her or escape. she silences you. You cant fight back. Fair warning. if your playing a LB u must ambush her extremely carefully. EXTREMELY . Kass also goes with that.
They are easily killable but you got make sure u burst them without them seeing you.
For instants. Stand in a Bush activate your W. Cast your Q ult at them. normally you would want to wait to cast your charm. but u need to use it. and get back into the bush till your W is back up unless you killed them already.
At this point in the game your W will deal like 45% damage with a 3 sec cd. unless they are tanks. witch u just got to kite them for a few W's. not hard.
Deadliest combo I can teach you. Use your W. Cast your ult once, to get all your W to hit them. cast your Q. more than likely they will start to run. If your doing this right. You can activate your W all 3 times you cast your ult witch is EXTREMELY deadly combo. save your charm for stalling for cooldowns or if they start to fight back.
As i said before. She is a deadly threat and since shes also a new chr, that also increases your chance to die. i average around 10-15 kills a game with about 15-20 assist with her. depending on how fast we win/lose. The deaths ive been averaging with her around 4-7. Not that im doing bad. I'm just getting blasted with ambush's and multiple ults. Also i tend to get greedy when I'm on a hot streak XD. Ahri ult is so bugy that you need to be careful when you use it to escape a lot of the time. so sec your stunned from a gang your done.
This Chr requires a lot of Safe playing. However your so strong it can be hard to. If your in a fight with someone and there's 8 minions in a group together. don't be afraid to use your Q on them to heal you instead of damage your enemy. do it right you could get them both. but like i said. Your W will be your game maker. I also dont think ive ever played a chr that ive gotten triple kill+ nearly every game. thats how efficient you can be with her.
any questions just ask me. I may not be a expert at her. but Im dam good at her for a chr thats only been out a short time. I dont play chrs i suck at tho :)
Also keep in mind that her ult prombly should never be relied on for damage. Chase/Run is what its best for. Its not vary strong. But its strong when used with the W and kited with it. :)
and im not saying my average kills is my highest kills with her! Thats just factoring in matchs end in 10 mins.

U don't farm in Dominion. U farm champions. Grab buff when u see it. Kill Minions when u see big groups of them. other than that get your gold from players not minions.

Pros:
Fast killer.
Herd Control.
Good range.
Great Survivability when ult works right. (careful around edges it will bug and cast in place)
Low cooldowns
Deals True damage
Has a Taunt
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Cons:
Runs out of mp WAY to easy. < even faster than V
Highly targeted
Ult bugs alot.
Q and E are skill shots. W autolocks to close's target. Requires timing
Commonly towerdived.
Vary Easy to kill.~` if in the wrong hands.