Glyphs

Greater Glyph of Scaling Ability Power vs Greater Glyph of Ability Power
It will take you until lvl 7 to get even in the AP and then outscale the Flat AP. Since you will be able to farm quite safely with Ahri's Q if you are in the need of waiting the time out, I would recommend the Scaling over the Flat AP.

Seals

Personally I like the Mana Regen on mids in general, mostly because Ahri can heal more often from passive if she is allowed to spam her spells but also because this makes you able to trade more in mid. If your opponent doesn't have Mana Regen you might even make them need to back early or take the harrash.

Greater Glyph of Scaling Mana Regeneration vs Greater Glyph of Mana Regeneration

Marks

Magic Pen is genius to have imo, you will often not rush a Magic Pen item (exept for Sorcerer's Shoes) making it worth to have some in your runes. This will also counter any early game items suchs as a Chalice of Harmony a bit that your opponent might rush.

Quintesses

Greater Quintessence of Scaling Ability Power vs Greater Quintessence of Ability Power
I find it better to take the Flat AP here, since it will take you until lvl 12 to have gotten more out of it. After that it will be 8.37 AP (2.79 AP per Quint) more if you choose to run Scaling AP, which I wouldn't consider the time waiting worth it.

Level 1 - Your First Spell

Q - Orb of Deception Most of the time I choose Q as my first spell. Pros: - Gives up to 80 damage, 40 of that being true damage. - Can damage all opponents within it's range giving a better start for jungler and clear in lane. Cons: - Hard to hit: Skillshot (though remember you might hit with its return!). - No CC. - Not the highest single target damage.

W - Fox-Fire I wouldn't recommend this in any way. Your W is hard to control if you don't have an engage or distraction (such as a jungler gank) going on. I would have done a pros, but there isn't really any reason what so ever to take it in level 1, but just to sum up: Cons: - No CC. - Hard to hit: Needs to be close before it targets champions and not go for a nearby creep. - Lowest single target damage at rank 1 (remember that your Q does ½ true damage)

E - Charm Your E is your other choice for your level 1 spell. This is your main CC but hard to hit aswell. I would recommend to take this only if you invade or are being invaded. This will not help your jungle either since a charm will reset the jungle creep, rather than give the damage needed in the pull at level 1. Pros: - Heavy CC for a level 1 spell. - Highest single target burst if you hit an AA afterwards (should be fairly easy to do so). Cons: - Hard to hit: Skillshot and doesn't move through mobs. - Resets jungle creeps (not the creeps in lane).

Leveling Up - Rest of the Game

Q - Orb of Deception Your Q is your main source of damage. Max it first. Pros: - Half of the damage is true damage - Can damage all opponents within it's range giving a lot of damage output and more potential heal from her passive. - Good wave clear. Cons: - Hard to hit: Skillshot (though remember you might hit with its return!). - No CC. - Not a direct burst, its the return that really hurts.

W - Fox-Fire Take this as the last spell. You need to get close before you can have a real impact in a fight with this, meaning it will be either hard to hit or hard to survive if you choose to get close. Properly your best burst spell but have a bad wave clear, making it hard to be able to leave your tower. Pros: - High Burst - Good defensive spell if engages upon - Easy hit if engaging directly towards an opponent Cons: - May choose to attack minions if close to them. - No CC. - Bad wave clear.

E - Charm Max this second. You're not maxing it for the damage but for the CC. At max rank it's up to 2 seconds where the enemy wont be able to move exept slowly towards you. You need damage for the CC to be worth it, which makes the reason to max it second. Pros: - Amazing CC Cons: - Hard to hit (can hit minions) - Resets jungle creeps (use only as last hit)

I'm not gonna explain Spirit Rush here.

WIP - Come back later :D

WIP - Come back later :D

Offensive Tree:

Sorcery - If you follow my build you wont get that much Cooldown Reduction from the items, also there is no real need for the other tier ones in this tree on Ahri.
Blast - More AP, why not?
Havoc - Giving that more damage output. Being that more of a threat as a carry.
Arcane Knowledge - Make their Magic Resist to no use.
Mental Force - Even more AP!
Spellsword - I like to take this making the Autoattacks count more when you're juking around the enemy. Making most of an impact in the lategame.
Archmage - Get that AP scaling!
Executioner - As an assassin this will help you burst down the enemies.

Utility Tree:

Summoner's Insight - Feel free to take this but only if you use flash. I will not take it because she kind of got 1-3 free flashes with her ult.
Wanderer - This is were I will personally use the free point from before. This helps roaming the map and help the other lanes.
Meditation - Mayby just me being a big fan of Mana Regen. Take this if you want the more sustain in lane, spamming spells more often and getting the passve used more.
Mastermind - In general a good spell, giving your summoner spells more presence in the game.
Runic Affinity - Get this to get the maximal potential out of your buffs (not counting Baron buff!).

(Might be deleted in this guides final form, no idea what to write here what so ever atm. Anything not covered though essential will be put here I believe)
WIP - Come back later :D