All of the Ahri builds I've seen haven't quite worked out for me the way I'd hoped, so I played this one, and it works for me every game. So I figured I'd share it with everyone!
Also, this is my first build guide...so please don't yell :3

Mid Ahri. Please.

The first thing my friends have questioned is my rune selection. I always tend to go more defensive in my rune selection, and its worked marvelously for Ahri, more so than anyone else I've played. This tends to give me great early sustenance, and I rarely need to stock up on many health potions or go back to heal.

I go with a basic 21/0/9 AP mastery. Self explanatory.

I think the item build is self explanatory as well. Sometimes I switch up how I get the items, but I always finish with the same build. If I'm doing really well or getting fed in mid, I tend to rush Ionian's before getting Tear. However if I'm being outplayed or just not doing so well I stick with Tear first. And I've always stuck to the rest of the build as is, unless being fed, then rush Rabcap.

I picked the Ionians over sorc shoes simply because of her ult...Morello's works just fine, but unless the opposing team is building magic resist, having the ~30 second cooldown on Spirit Rush is very nice indeed.

(I don't know why the thinger is saying you get 420 AP...last match I played I had 723 with full build and Archangel's proc wasn't even maxed yet)

That's all there is to it. Just wanted to get an effective build for her out there, because Ahri is definitely one of the most fun champions to play, as long as you can line up her skill shots. But hey, Spirit Rush is perfect for that