Hello, I am hardRAWKR. I am a strong player in every role, and mid is no exception. This guide says Ahri on it, but this is my build for quite a few mids, such as Orianna, Zyra, Cassiopeia, etc. It is extremely strong early and mid game, and if you are able to get kills and farm it should scale into late game well. It lacks mana regeneration, however, so it is extremely important if you are on a very mana-heavy champion to get blue as often as possible.
This is my first build on mobafire, so I hope you like it.

I run a pretty basic rune setup. You can substitute the seals with mana regen/lvl if you want. I run flat because I share this page for all of my mana mids, and Orianna, who I play frequently, really really needs the mana regen early on.

Pretty standard 21-0-9. I think that this is absolutely the best mastery setup for any mana-based ap carry.

This is where my build strays from a lot of builds I see. You can start with Boots+3 Health Potions or a Doran's Ring. Boots are the safer option, but Doran's Ring will make you stronger in lane until your first back. Sometimes I even buy Boots, 1 Sight Ward, and 1 Health Potion if I expect an early gank. It depends on the matchup and how you intend to play.

On your first back, you should have enough gold for 2 Doran's (or Doran's and Boots). If you have any extra money, buy a ward or 2. These will keep you safe from ganks. Very important.

Get your Sorceror's Shoes next, and then Kage's Lucky Pick so that the GP5 can start ticking. Then get Haunting Guise.

Haunting Guise is an interesting item. It doesn't give much AP, but it gives 2 of the strongest stats in the game: Health and Magic Penetration. With just Sorceror's Shoes and a Haunting Guise, you penetrate through 40 Magic Resist, which will put most squishies at 0 Resist, and the Penetration from Haunting Guise is actually better than a Void Staff until the opponent starts building MR. At the price of only 1485, Haunting Guise is a steal.
After that I finish the Deathfire Grasp so I can be super bursty. I find that DFG synergizes particularly well with Haunting Guise, because the Magic Penetration means that DFG is going to do pretty much the full % of damage from your AP.

Next, I get Abyssal Scepter to continue the theme of ignoring the opponents' Magic Resist. I prefer Abyssal over Void Staff because since I already have a lot of Magic Penetration, having the 40% from Void Staff is overkill in most cases. Although it costs more money, the stats are better and you should have plenty of gold from all the kills, farm, and the money from your Kage's Lucky Pick.

Now you can start selling things to get your final items. Usually, I get my Rylai's before my Deathcap because the slow is extremely powerful in teamfights. However, on some champions, particularly, I get a Deathcap and skip the Rylai's entirely because she already has plenty of aoe crowd control. It really comes down to personal preference. Don't sell your Haunting Guise until you get Void Staff or you will notice your damage drop. If you can find room, make your last item a Guardian Angel. If you feel that you're getting focused down in fights, you should probably get a GA earlier, but in most cases I'm able to stay outside the fight without dying.

This skill order is obviously specific to Ahri. Pretty similar to most Ahri builds I've seen. I like to take Charm at lvl 2 and wait until 4 for Fox Fire. This allows me to be very aggressive starting at lvl 2, which is exactly how Ahri should be played. If the opponent is putting on a lot of pressure and pushing to tower, I usually take Fox Fire at lvl 2 instead because it allows me to farm under tower more easily. Other than that, I use the same skill order every time.

Flash Ignite. Pretty standard.

These pro/cons will be regarding the build, not the champion Ahri.

PROs
-Extremely strong burst early/mid game.
-Not very costly early on.
-Works on many different champions.

CONs
-Bad without runes and masteries because your AP will be very low until late game and you might have mana problems on certain champions.
-Your item slots fill up quickly so it can be hard to squeeze in items sometimes.
-You will fall off late game if you don't do well.

FARM FARM FARM FARM FARM
PLEASE FARM
DO IT
FARMING IS BETTER THAN GETTING KILLS

In almost every game that doesn't have a Mordekaiser (huehuehuehue) in it, I have a solid farm lead on everyone else in the game, which definitely translates into a gold lead. Although getting kills is good, farm is the best, most consistent source of gold in the game, and it's very easy to do. Starting as soon as the lane phase ends, if a lane is pushed even a little bit to your side, go farm it. This applies to any hero in any role. Farm farm farm farm farm farm.

Follow this guide and play smart, and you will do great. That's about it.

Rate/Thumbs Up/Like or whatever.

:D