This is my first build I'm posting, I may have messed up a bit, don't troll too hard because of it.
Other than that, this build has been working insanely well for me, I don't recommend playing Ahri if you can't hit skill shots, Being able to hit a skill shot every time is extremely crucial to living, farming, and of course, carrying your team that will most likely be filled with feeders.
Mid isn't bad if you just want to farm all day because other than LeBlanc, nobody can really outzone you if you constantly harass with your "Q" her mana costs are low except for her ult, and the Meki Pendant combined with your passive lets you lane extremely well.

I go with these skills because it allows me good harass and at level 2 you can completely dominate those annoying Shaco that love to try and gank you, just give them a kiss and in the turret they go to meet their death while your "Q" heals any damage his shiv may have done, providing you have harassed enough to get your passive to 9 stacks.

I don't like to max out fox fire first for one reason, and 1 reason only, too short of range. You can just as easily hit your enemy with your "Q" if not more easily, considering it goes through minions and the timing is your own. I always max out my "Q" first even with the low ratios you will be hitting harder with that than you will with Fox Fire because of the true damage on the way back, if the excuse you use for not maxing out "Q" first is because it's a skill shot, then you shouldn't be playing Ahri.

Charm I max out last, even though it does scale pretty well IMO it just doesn't have the required burst to get in, finish them off, and then get out either with ult or Flash, if you are going against champions that you have difficulty with such as Pantheon, you may want to get your "E" to level 2 or 3 before getting fox fire past 2 so that you can prevent spears, stuns, and other harass, also, it makes getting them in turret range much easier.

I think the Runes I chose are pretty much self-explanatory, with the exception of the seals...many people will wonder "Y U DO DIS!?" And I reply simply, "Because I can turret dive and live, while you die before you have a chance to get off your first attack" :)
Cooldown Glyphs prevent me from having to build any further cooldown items and as long as you know how to farm then you shouldn't need any AP runes because you will be able to buy items which are 20x better plus, M-PEN runes are so much more godly.

Now to items:

• Meki Pendant - I start with this to later build into a Morello's Evil Tome , granting me cooldown reduction, plus, the mana regen makes laning extremely sustainable, as long as you don't mind a little risky play to activate your passive time and time again.
• Rod of Ages - The reason I get this instead of Archangels like many others do...simple Ahri DOESN'T RUN LOW ON MANA, However, she is EXTREMELY squishy, plus, I like to think RoA stacks better with Rabadon's Deathcap .
• Sorcerer's Shoes - Simple, Magic Penetration (M-PEN), honestly...I can't think of getting anything else, her cooldowns are already rapid after Morello's Evil Tome, so there is really no need for Ionian Boots of Lucidity.
• Fiendish Codex - I obviously build this into Morello's Evil Tome later, however I get it now because I am going to start needing the cooldown reduction for my ult, and Charm.
• Hextech Revolver - I don't finish my WotA because I really need Rabadon's Deathcap at this point because the enemy champs are going to have so much health your burst wont do enough and they will have the M-RES to counter you if you don't have the M-PEN to counter it.
• Rabadon's Deathcap - Simple, it's OP for any AP carry/assassin in fact, if you get fed enough, get this before RoA and they wont have the chance to do enough damage hence you wont need RoA immediately, but by NO means decide you don't need RoA at all, it may be more of a core item than boots for Singed.
• Morello's Evil Tome & Will of the Ancients - I already explained this, if you REALLY need to know why...well then, I'll tell you, Cooldown, Mana Regen, Extra AP + Allies get extra AP, Spell Vamp (AKA after 9 spells you become full health no matter how low you were previously)
  
  Yes, I know I didn't include a 6th item, if you are getting focused, find out what damage is doing the most, if your team is bad, get a Thornmail so you, the AP carry can backdoor turrets, no, I'm not trolling right now, I've had to do it with AP carries before, and have won games because of it. Guardian Angel is sometimes funny to use if you are trolling them and have zero deaths, and late game, want to keep it that way if you get into a tight spot from getting overly confident in Ahri's godlike powers.

E > Q > W > R > Q > W > R > R >
That is, Charm > Orb > Fox Fire > Ult in (AFTER FOXFIRE IS ACTIVE W/CHARGES) > Orb > Fox Fire > Ult (If target is dead use your 2nd and 3rd charge to either farm surrounding minions, go for another kill, or run from other enemies) > Rinse & Repeat (Wait for Cooldowns/Health/Mana - GO GET YOUR PENTA!)

I know it definitely isn't the best guide, mainly wanted to get my basic build out there because I'm sick of seeing archangels everywhere, kind of makes me want to cry. Any constructive criticism is useful, troll if you must because I can care less how cool you think you might be and how much "better" than me you are.
Have a nice day, hope you like the build, give me some responses. :)

PS. Use smartcast, I use it on everything except my charm because of mainly habit, however, seeing as it is your ONLY CC then I would HIGHLY recommend you don't mess up because of smartcasting, if you aren't used to smart casting, now is the time, skillshot champions make it almost like using targeted spells, if you think you can hit your skills fast enough, then by all means do so, but trust me, smart cast is your best friend.