This is my first guide on mobafire, so don't be harsh on me. Why I posted this build? so you guys can profit. I think this is a pretty decent build, so I wanted to share it with everybody.

for runes i take:
9x Greater Mark of Insight
9x Greater Seal of Potency
9x Greater Glyph of Potency
3x Greater Quintessence of Potency
Basically Flat AP, flat Magic pen, so your damage is pretty well thoughout the game. You can also take Ability power/level runes, or mana regen/5 runes.

Offense Tree: (More Damage)
4/4 Mental Force , Ability Power, for the extra damage
4/4 Sorcery, more Cooldown Reduction means faster spamming Abilities
1/1 Arcane Knowledge , Magic Penetration, for extra damage
3/3 Havoc , Increases Damage Dealt, seems obvious to me
4/4 Blast , AP/level, even more power :)
4/4[[Archmage], 5% more Ability Power means significant damage increase
1/1 Executioner , For easier kills
Defense Tree: (More Survivability)
2/3 Resistance , A bit more magic resistance
2/3[[Hardiness], A bit more armor
4/4[[Durability], More health/level
1/1[[Veteran's Scars], More Health

2 Doran's Ring to start with, then Boots so you can escape/harass easier, Catalyst the Protector for some more survivability and mana (because you will get low on mana early game, when you try go get the enemy low on hp). then Hextech Revolver so your passive becomes usefull, and your normal spells will heal you for a bit too, plus the extra AP, which is always good. Sorcerer's Shoes for magic penetration, you can also grab Mercury's Threads if your enemy team has a lot of CC. Rod of Ages, health, mana and ability power, awesome! Needlessy Large Rod +80 AP, to give you more damage output. Will of the Ancients for decent spell vamp, and you'll help your team too by giving them AP and spell vamp. Rabadon's Deathcap, Obviously for the enormous AP boost it'll give you. now you sell one of your two Doran's Ring and buy a Void Staff (AP + Magic penetration). if you have money left, sell the Doran's Ring You have left and buy a Rylai's Crystal Scepter for the Health and the AP.

Other Items:
Abyssal Mask, AP and Magic Resist
Lich Bane, Others think this is a pretty good item for ahri, i just don't like it that much.
Morello's Evil Tome, Cooldown Reduction and pretty sweet amount of AP.
Zhonya's Hourglass, Invulnerable for 2 seconds and Armor + AP. good if enemy team has a strong AD champion

Fox-Fire's (W) power is heavily underestimated by most enemies. but if you use it and Dash to them with Spirit Rush (R), you will do quite a bit of damage. Orb of Deception (Q) does not do tons of damage, but pretty much. it is especially well against enemies who are stacking Magic Resist, because when it goes back it deals True Damage instead of Magic Damage. if you land Charm (E) first, you are ensured that Orb of Deception will hit. My standard sequence is Charm, then Orb of Deception, then you activate Fox-Fire and dash after your running enemy with Spirit Rush. if you are in a teamfight, try to get one enemy out of the group with Charm, then make your team focus him/her. if you can't get one out of the group, just keep spamming Orb of Deception and Fox-Fire till they have low hp, then dive in with Spirit Rush

I just take Ghost and Flash, so you can easily get away, but you must choose the summoner spells that suit you best.
a list of all summoner spells and their score:
Good choice, Enemies movement and damage reduced
Good choice,Pretty good for Pushing and Defending
Good choice, for ensuring kills and denying heals
Good choice, For when you need to escape and your ulti is on cooldown/ or you are below level 6
Pretty good choice, To escape quickly
Not Good/Not Bad, Not my favourite
Not Good/Not Bad, extra AP is useful, extra attack speed is not
Not Good/Not Bad, never saw anyone use it though
Not Good/Not Bad, good early game, mid/late game useless (you have lots of mana)
Not Good/Not Bad,only pick if your team doesn't, and you need it
Bad choice, you're not a jungler
Bad choice, useless in my opinion
Very Bad choice, 9 min cooldown

to wrap it up, Ahri is a fun champ to play, and pretty powerful at the moment. i'm going pretty well with this build, as you can see in the picture:

hope you guys play better now you have this build :)

Pros:
- High Damage Output
- Sexy Champion
- Ranged taunt
- True damage on Orb of Deception
- Dash which you can use 3 times in a row, so you can ensure a kill or get out of a teamfight.
- Fun to play
Cons:
- Targeted first in most team fights
- Skillshots ( Orb of Deception and Charm) are rather slow

Early/Mid Game: just keep last hitting with auto-attack
Late Game: cast your Orb of Deception over a minion wave, and if you have enough power you kill them with one spell :) if you have not enough power you cast Orb of Deception again.