Very generic. I opt for the 21 points in offense taking all the AP masteries. In utility it seems mana regen and increased buff duration would be best

Magic Pen marks are pretty generic and essential considering most of your damage is magic-based.

Ap/level glyphs are probably best because they give a very nice boost late game.

Flat AP quints for that early game damage

Mp5 or Mp5/level is essential for mana regen. Mp5 is good up until level 4. Choose one depending on your playstyle.

BEST CHOICES

Ignite is a must to get those kills early game and it scales decently into late game as well.

Ghost is a very nice choice considering you have 3 flashes on your ultimate and movement speed seems to be essential on Ahri.

Optional Summoners

Flash is good to use to get away from those early ganks.

Cleanse is a very nice spell after the buff. CC is your worst enemy.

Maxing foxfire first seems to be the best way to go, as the mana stays the same at all levels and it reduces the cooldown per level up. However, I max this SECOND because Orb of Deception seems to be the best for laning and harrassing as well as synergizing well with your passive.

I max charm LAST because a measly 0.25 charm duration per level isnt that amazing. Also the mana cost rises a lot and it just isnt that strong of a nuke and won't provide you with the most damage output

Obviously, put points in your ultimate whenever possible.

Starting paths: Boots + 3 hp potions OR mana crystal + 2 hp potions

First trip back: Catalyst + wards n' potions (+sorc shoes if you got a kill or something)

First big item: I'd suggest [Rod of Ages].

Core Build:

Rabadon's Deathcap, Rod of Ages, Sorcerer's shoes, Lich Bane

Situational items

Void staff - if they grab MR
Banshee's - if their AP carry is nuking you down too hard
Zhonya's - great active and good if you are getting raped by their bruisers or AD
Rylai's - To be like Phreak

Obviously i havent played her yet so this is just a rough guide...and my first guide lol

Anywayyyy leave a rating and if this guide gets anywhere Ill definitely add in some more stuff since I'm buying Ahri the moment she comes out =)