I've been playing Ahri since the day she came out and I do not like the way that Phreak and others suggest to play her, which is why I went totally opposite in the mastery tree. I have play this way through 20+ games and domninate my lane other Ahri's and the other team. Take it as you want.

Greater Mark of insight: Simple enough magic pen everyones needs it.

Greater Seal of Replishment: Helps sustain early game without having to base very often.
Pair this along with the meditation mastery plus Dorian's Ring early game and you regen about 10-12 mana per 5 sec.

Greater Glyph of Shielding: Magic resist for early to late game

Greater Quintessence of Insight: More magic pen nom nom everyone!

I do not like the idea of 21/0/9 as suggested by Phreak, however I chose the opposite route as 9/0/21. I chose this mainly for incrased cooldowns on abilities, however the benefit of increased magic pen in the offensive tree as well as the spell vamp in the utility tree are a nice bounus. So far has proven to dominate everyone even other Ahri's, who I can only assume went with what Phreak built easily all day any day.

Will of Ancients: decent AP but most importantly lifesteal!!!!!! I built this first it is a must in my opinion, once you have this you dominate your lane hands down. What's that you say???? 28% lifesteal at lvl 5 omg I try and I try and I try to hurt Ahri but she just heals back up omfg so OP.

Ionian boots of lucidity: Hey even more cooldown 25% once this is built. Pair this with blue buff omg 40% cooldown, whats that my Q,W are back every 2 sec, my ulti back in 35 sec omfg OP.

Rylai's Crystal Scepter: A must for this char. The hp, ap buff alone make it great but what makes this even more awesome is the fact that it works well with your W ability. What's that you have a rampaging Volibear coming at you no worries just hit your W and its like bear mace to his face. As he becomes slowed beyond belief from the burning sensation in his eyes you just walk away on your merry way.

Void Staff: 40% magic pen omg why wouldn't you have this. Your ulti now shreds anyone and everyone congrats you just caused the other team to surrender.

Deathcap: Increases ability power beyond belief. When this is built I have around 500 AP.

Archangels Staff: Increase ability beyond belief. 650 AP when this is built and maxxed. Quaff a blue potino make that 700 AP take baron and you are closer to 800 AP.

Rod of Ages: I've started using this for the health and mana increase puts me around 3000+ hps godly.

*Note*

I've been seeing a lot of people built soulstealer on her, with my build there's no need. Archangel gives you more AP more mana, plus you don't lose the AP or the extra CD when you die.

If you get to the point to where you built archangels and max it you pretty much will destroy everyone. Your ulti and foxes will do so much damage that low hps chars like teemo ashe talon misfortune etc.. instadie everyone else takes a second or two longer.

Eh I leave this up to everyone to decide on their own. I do not choose my E till like lvl 12 because it is a difficult shot to land and doesn't do enough damage early game to be effective. I choose to max out my Q and W and R as much as I can before I have to choose it.

Ignite/Exhaust only two I ever use. Everything else is a waste to me..... personal choice.

I'm not sure how much more detail I need to put into this I guess I can if people want it. But as I stated earlier this is my build and works well for me. Ahri is a very fluid champ that to me isn't difficult to play as if one can figure out the fluidity of her abilities, however others may disagree but whatever. Anyways here you go!!!! If I see you out on the battlefield and rape your face with this char don't be mad, my build is just better.