Well, this being my first guide don't be too hard on it. However, after looking around I could not find an Ahri guide that I would consider complete. As such, here is my take on Ahri where you will hopefully find a generic build which you can adapt to your own style and liking.
Let's start with everyone's favorite section to complain about.Yes I am omitting some "key" runes and on top of that, I am taking runes that other guides clearly classify as "useless" for Ahri. Well, I'll say it here: THEY ARE WRONG! Here is why...
First, the Quints. Yeah yeah yeah cant play AP without the flat AP quints. That is horse poop. Ahri is much closer to an assassin then she is to a pure AP caster. As such, she needs SPEED to survive. The extra 15 AP early on is nice I agree....but late game having that bonus movement speed is too good when trying to chase someone down or get away alive. To top it off, most of Ahri's skills are somewhere on the order of 0.3 bonus damage based on AP....meaning you get about 5 of the 15 bonus AP to actually count...in contrast to the extra 4.5 % movement speed...You do the math here, but in my opinion having that speed that an assassin needs is more essential then 5 more damage.
Next is the Glyphs. In my HUMBLE opinion. Ahri has the BEST ult of all time. This means you just HAVE to abuse it. Escaping, chasing just bursting this ult that can be cast 3 times in a row has an unfortunate 60+ sec cooldown. As such, shredding even a second off of this amazing skill is worth the effort. Hence the CDR Glyphs. However, my main reason for getting these is in part due to the fact that you are only getting the boots for CDR. Many other guides suggest that Evil Tome....but no matter how many games I played with Ahri, the Lich Bane always came out on top of the Evil Tome...except the nice ability to spam your skills. Therefore, by getting CDR Glyphs, I can more or less cut the Evil Tome yet have almost the exact same ability to spam skills. Combined with a blue buff and/or a blue potion and you are a machine gun of deadliness.
The last two, Seals and Marks, I really didn't get to experiment with too much. In part because they really don't have too much impact on almost any champion's game play...in my opinion anyhow. As such, since you aren't getting those AP quints, this is where you can compensate. It has almost been established as fact on these forums that AP per level are vastly superior to flats in this category so I will not spend anytime proving this to you...go read other guides and you will understand why since my main point here is to focus on the champion, not superiority of runes.
The masteries that I recommend are practically a staple 21/0/9 which are almost set in stone for AP assassins. The only difference could be in that 9 in utility. As mentioned before, Ahri NEEDS and I mean ABSOLUTELY NEEDS movement speed. Her base 305 is pitiable for an assassin, meaning if shes being chased and the ult is on CD...well lets say probably won't end too well for her...The other "unusual" choice is the extended buff duration. Now, I don't know if I have mentioned this or not but Ahri is a HUGE mana burner. Her abilities are so spammable that before long you find yourself depleted of mana. Thus, if you get lucky enough to obtain a blue buff, having those extra couple of seconds of mana regen could be vital in a team fight or simply laning.
This section I must admit is not quite where I want it to be. More precisely I don't really have a level by level breakdown of her skill acquisition. However, here is my priority when getting skills:
Basically, whenever I can get the lowest possible number on this list I do so. This means that once again, Fox Fire is maxed last after maxing Q then E. The reason for this is obvious in my opinion. Q is Ahri's best nuke (besides her ult) dealing TRUE damage which means you HURT anyone you want no matter how MR stacked they may be. This however does come at a price. This is a skill shot. This means that although the damage is nice, it only happens if you land your skills. In fact, the true damage is only on the way back to you so try to position yourself so that at least that return trip hits the target.
The second skill I max is E, charm. On top of being cute, this in my opinion is like the most OP CC skill any assassin has in this game. Besides doing a substantial amount of damage, the target simply walks towards you for up to 2 seconds AND is slowed. This means that not only can they not run away, attack, skill or use summoner spells (besides cleanse), but you also get to land your Orb of Deception and Fox Fires without much effort. The only downside is...it is a skill shot AND hits the first thing it can. Minions are your ultimate evil counter against this skill.
I max Fox Fire last for a few reasons. Yes it isn't a skill shot and yes it has some very nice damage... however there are drawbacks. First, this skill is rather slow in my opinion. When you activate it, the first thing that happens is the three flames appear around you. This might seem like a trivial bit of time, but when the fight comes down to 50 HP per person, the one that can pull out their skill faster will win... which will not be you if you use fox fire. Next is the auto target of the Fox Fire. The skill hit the closest 3 targets, meaning if the enemy is near their creeps, well you'd be lucky to land even 1 of those fox fires on the champion...
The ultimate, Spirit Rush, is like mentioned before, one of the BEST in the whole game. This alone is like 3 flashes combined with some very nice damage. Oh, did I mention that this skill hits MULTIPLE targets? This means if there are some enemies that are almost dead and are running, you can pick up some very nice and much needed gold very fast. Even if they try to turn around and fight you, just dodge their feeble attempts while dishing out even more punishment. But more on that later.
The item build in this section is for the classic, 5 v 5 situation. Also, chances are you will NOT finish this build most of the time, but in case you do, it will be rather rewarding. Also, this build is NOT set in stone, and like any good player, you MUST adapt to what your enemy is building. However, i will point out key items that you should use regardless of what the enemy is building.
Opening with boots and 3 HP potions, in my opinion AND many other more experienced player's opinions...is the SAFEST opening. If you are matched up against something like Anivia or Brand then it would be foolish NOT to open this way (dodging those skill shots IS hard without boots) but even if going against non skill shot based champions, the boots will help you live thru ganks AND will help you land YOUR skill shots. Ahri is very dependent on her positioning. Boots help you get to those sweet spots that would deal optimal damage to your enemy.
Next is my personal choice, the Catalyst. As I mentioned before, Ahri burns thru mana VERY fast. Since masteries and opening items+runes don't really give you that MP regen (and they shouldn't, you are not a support) you need some sort of sustain item. This is where the catalyst shines. A very healthy HP and MP pool on top of significant regen of HP and MP upon leveling, I don't see why you WOULDN'T want that. This alone can keep you in lane for way longer then any other item.
At this point, the next time you go back to base you have several choices. If you see that the enemy champion has gotten those upgraded boots or they are simply much faster then you, you really should get your ionian boots despite how appealing a little more damage oriented item may seem. The CDR + extra mobility will increase your survivability significantly even though your DPS is nothing special yet..
At this point, the most variation in your build should occur. Depending on how well you are doing you can assemble an appropriately priced item. If you are having a hard time laning and finding yourself going back a LOT to heal, take the revolver. If you are dominating or simply keeping up and farming undisturbed, go ahead and finish that rod of ages. Past this point should be mid to late game. The most potential for profit AND variation in your build. You COULD follow all my suggested items in that order, but once again this is very situational. If your opponent team is stacking some heavy MR, consider getting a void staff right after your Hextech Revolver going straight into the Deathcap. If you are just dying way too much because the other team is full of jerks that focus you all the time ( as they SHOULD if they know whats good for them...) get a GA so that even though they burst you down, your team can start killing them, you revive and get your revenge. In both of these situations, consider skipping the Rylai's or later exchanging RoA for a Rylai's.
Ahri is an assasin. This means you are playing, in my opinion, the hardest but most rewarding class in the game. Your early game should be farm farm farm. Ahri is VERY item dependent and NEEDS cash. However, even early on you can deal some very nice damage on to more squishy targets. If a wild Udyr or Rammus appear, DO NOT FIGHT THEM if they are full or almost full on HP. Ahri's greatest weakness is a bulky DPS with some CC. However, if the opposite is true and that same Udyr or Rammus are just a hit or two away from death and you have 1/2+ max HP, dont hesitate to turret dive and pick up the kill with your ult. Ignite + 1 ult would usually do the job and you will take at most 2 turret shots. However, be smart as with any character, turret diving at low levels VS. multiple enemies that are either damage heavy or have CC's is suicidal.
Mid to late game, when team fighting is or rather SHOULD be the only time you fight someone and commit, you have a very important role. Most of your team will likely hit a road block, namely that annoying Rammus, Udyr, Chogath etc. while the enemy DPS is shredding your team. This is where you wait for their DPS to unload on YOUR tanks, and then dive in with your ult past those annoying tanks. However, try to go AROUND the enemy team harassing INTO them rather then diving into the middle of the fight. You are an assassin, and not a very tanky one at that so 1 CC that hits you will probably be all that the enemy needs to kill you. Your priority should always be the squishiest, furthest back nuker that you can burst down in 1 combo. Chances are, something like a Veigar will deal WAY more damage then any AD carry WAY faster, so the quicker you kill something like that, the more chances your team has of winning the fight. However, that being said, Ahri is rather a team dependent champion. If you are stuck with a bunch of feeders and no actual tank, then sometimes you will have to initiate a fight. This fortunately is rather easy to do. Wait for that cocky ranged ad or ap to sneak out for a harassing hit or two and pop your ult into an E. BAM! you just pulled out a squishy DPS from their team and if you have more then idiots on your team, that squishy is dead before the fight even begins. Even if the enemy decides to retaliate, this has become a 4 v 5 which you should have a much better chance of winning.
Well, this is about all I wanted to discuss about Ahri at this point. I know this guide is NOT very reader friendly and lacks any sort of visual aid. But as I have mentioned before, this is my very first guide so I do not yet know all the ins and outs of this interface. However, the guide itself I do hope that you guys (and ladies) find at least somewhat helpful. Also, I am open to CONSTRUCTIVE critique. Saying "YOU ARE AN IDIOT AND YOUR GUIDE IS RUBBISH!" without saying WHY it is so doesn't help me get better nor you to prove why your right. I will try to read any comments you guys leave me and update the guide as the times goes on. However, for my last remark I must mention this: Ahri is incredibly fun yet difficult character to play. Therefore, the only REAL advice I can give you is to practice playing Ahri, don't give up and feel free to be creative with her builds. There is no one way to play ANY champion and Ahri is no exception.