First off, let me say Cho is a metric piss ton of fun to play, whether you build tanky or as a caster. This build is for those who want high damage and some survivability, and it works quite well for me. My Cho is far more AP than tanky, mostly because I feel he lacks the ability to initiate well, nor do his abilities benefit from tank items. Tank Cho is a big target, that can be ignored once he chomps someone. The benefits of playing him as AP vs tank is that AP scales great with his abilities, and your damage is absolutely lethal. You do need some items that give you survivability, to live through being focused down, and not just be a giant glass cannon. Hopefully this guide will help you utilize Cho's skills, and with that be an asset to almost any team composition.
Edit: Even Cho gets hit by the nerf bat. Or so it seems. Feast was hit, the base damage reduced from 300/550/800 to 300/475/650. Yikes. But, surprise surprise, the AP ratio was incresed, from 50% to 70%. Huh. Feral Scream damage was nerfed, ( to 75/125/175/225/275 from 80/130/180/240/300) but Vorpal Spikes was increased:to 20/35/50/65/80 from 20/30/40/50/60, and the ap ratio bvuffed, to 30% to 20%. So what does this mean? Tank Cho is now even more of a joke. He will not get the damage to support it. AP mage cho, on the other hand, is looking more viable. The hit to Feast does hurt, mostly because you will have to rock around 400 ap to get to 800 damage with feast, and ~700ap to out do the damage you were originally getting.
So, we will see.

Runes that fit any caster. Cool Down blues, Mp5/lvl yellows, MP reds. As for quints, I like base mana, or mp5 flat.
Also to consider: MR flat runes. I'm going to save to get these, and edit this guide later if I like them.

I don't recommend health runes of any sort on Cho. He has a built in way to regain health, and he just gains more overall health with his stacks. I feel that runes that buff hp are unnecessary because of this. I often see people throw as much hp on the guy as possible, ( warmogs everywheres) but it's useless if enemies can tear through it anyway because you have no resistance.

I run ignite/ghost. Cho is slow, and lacks a way to escape or chase. Ghost makes up for that. Ignite is for the extra damage,especially early game, because sometimes you can't outright drop an enemy.

Other alternative spells:

Teleport: Awesome skill on almost anyone. Teleport to a ward on a bush for a gank, tele to a lane getting pushed, tele to shrooms, it's good for getting somewhere quickly.
Exhaust: An extra slow.
Flash: Another escape, or a alternative chase.

Maybes:
Cleanse: I don't run it, but if you know you're against a lot of CC, it might be useful.
Clairvoyance: Useful in that it helps you spot ganks, enemy jungle, etc, but I feel situational depending on your team.

NO:
Fortify: Not helping you live, not helping you kill.
Heal: Lame, he has a mini heal in his passive.
Clarity: Meh. This is what runes and items are for. Learn how to conserve mana if it's that big of an issue.
Revive: You are Cho, and as such, you shouldn't be dying so often that you even need this. In fact, I don't believe anyone runs this in seriousness.
Smite: Are you kidding me?
Rally: HAHAHAHAHAHAHAA

I tend to run 9/21/0. Defensive tree is a must. The added MR and Armor will help you survive early game, and will make the difference later on when foes are trying to beat down the massive feasting monster that you've become. For the other 9, I tend to like put them into offense. The extra bit to pen is good, but more important is the Cool Down reduction. You want feast as often as possible, not to mention rupture and feral scream.

Cho's passive is why I don't start with health pots nor worry about health runes. If you are good with last hitting, it will let you stay in lane far longer than your enemies, therefore getting more exp and gold.

Put rupture up as soon as possible. Put a point into scream and vorpal spikes at 2 and 3, obviously your ulti when possible, but Rupture should be leveled first. It has a 100% AP ratio, is both your slow and your damage, as well as on a reasonably short cool down. Plus it's an AOE. It disorients enemies by popping them in the air, slows by 60%, and you can hit multiple people. It is Cho's best skill.

Feral Scream is the next skill to get points when you cannot level Rupture. The AP ratio on this is %70, plus it's an AOE silence. Great for team fights, and the range is a little bit longer than the animation, so sometimes you can get that kill on the enemy running from you.

Vorpal Spikes are pretty much for farm. The ratio on them is 30%, so while small compared to your other skills, still helpful none the less. The only reason to toggle them off is if you are wailing on a tower and an enemy is cowering behind it. You don't want to take unnecessary tower shots because your spikes hit an enemy who wasn't going to attack you anyway.

Last but not least: Feast. The first thing to understand about feast is that it deals True Damage. Meaning it ignores MR. With AP Cho, some enemies will try to build MR to counteract you. Feast looks at their MR and goes "psssssch, and?" while destroying them anyway. Keep in mind that 70% of AP is added to Feast's damage. Feast is how you murder people.
Not only that, it rewards you for killing with it by giving you more health, and making you bigger. Capping at 6 stacks, of course. Keep in mind, that if no champs are around to easily kill, feast minions, especially when you first have feast and are just getting into your build.

Alrighty then, now down to the reason anyone ever reads guides: Item builds.

Start off with a meki pendent and two mana pots. At least for me, I tend to harass with Rupture, and even with runes Cho is a bit of an early game mana *****. The pendent helps. Health pots are really not needed off the bat, but if you find yourself recalling with some extra gold, I recommend grabbing a health pot pot or mana pot (whatever you feel like you need more), plus a ward if you need it. Grab an amp tome soon as you can, which with the meki pendent builds into a fiendish codex. This is almost always the item I rush on Cho, as it is both AP and Cool Down reduction. It also later build into Deathfire's Grasp.
After the Codex, depending on how well you're doing, finish Deathfire's,(Why Deathfire's? CD reduction, AP,and a built in nuke.) Consider grabbing a Void Staff for AP and MP.
Alternative for the mana hungry: Catalyst the Protector into Rod of Ages. Health, Mana, and AP, it will in total give you 100ap. Keep in mind that you should get it reasonably early so it can stack. (needs 10 minutes.) The bonus health while good, is something that you don't always need (situational) and the same with the mana. As well, it cost more than a void staff or abyssal scepter, but you won't see the full benefit right away. More health doesn't equal armor or MR. I haven't ever needed the extra mana either, but, it might fit your play style.

If you are coming out worse for wear in team fights, get something that will help you live. Guardian's Angel is what I like against teams with high burst damage/AD, it gives you MR, armor, and lets you keep your stacks in some situations. Abyssal Scepter is more of an ideal, it gives you MR, AP and lowers the MR of enemy champs.

Boots: At some point by now you should have picked up boots of speed. Finish these into either boots of Swiftness for chase, Merc Treds if needed, or Ionian boots of Lucidity if you need more CD and especially if you haven't finished Deathfire's yet.

Rylai's is my favorite item for mages. It gives health, AP and a slow, which for Cho means you hurt them more and catch up to them quicker. What's not to love?
Alternative for those who keep dying: Frozen Heart. More mana, more armor, also a slow. Not ideal.
Alternative Alternative: Force of Nature. MR/health regen if you can't seem to live.
ALternative another: Zhona's Hourglass.


Late game, and you're kicking serious ***? Rabadon's Deathcap, as if you weren't killing them hard enough already.
Alternative late game : Zhona's Hourglass.

There are tons of things that go into being a good player, i.e last hitting, zone control, team fight positioning, etc. I'm going to hope the player reading this understand those,I'll gloss over what I can and try give you a basic idea of what you're supposed to be to your team.

1.)AP Cho is a mage, not the main tank. He can off tank, but Cho cannot be the only thing that can take a hit on the team, it just will not work. I don't care how good you think you are at tanking with Cho, he will drop like a fly if you rush into team fights like you are Rammus, especially with this build. If you plan to tank with Cho, be prepared to be ignored as you chomp once per team fight due to 60 second CD's on your ult.
"Alright Ailyen, I can't really run in there, initiate, protect the squishys, and expect to live. That doesn't sound fun at all. So what do you mean by 'off tank'?"
You will usually make it out of team fights with over half health, when the carries are on their last legs, running for their lives, and the tank on your team dropped. It is when off tank Cho hovers behind the carries as they flee, throwing down a silence and rupture that will deter enemies from chasing. Especially because you might have just ate the enemy tank. Off tank Cho charges in behind the main tank, and destroys who ever happened to tick him off. Off tank means you can take a hit or two, and still have enough health to stay in a fight. You can start a fight, but be careful of where you are and what allies you have with you, before you commit to 3 v 2ing the guys trying to back door or whatever.

2.)AP Cho should try and combo. After your tank initiates a team fight, you focus that sonofa***** enemy carry who has been dropping your ashe or mal. Drop Rupture, Silence as many people as possible, and eat the ***hat. He will die, and if he doesn't he is running back to base as you took out 90% of his health before he could even roll the keyboard. So why shouldn't you initiate a team fight with this? Good players can easily dodge rupture,and that is essential to your combo. You blow that, and they run the other way while your slow *** is trying to catch them. And if you even consider chasing enemies back to their base without your team, you deserve the painful death that awaits you. You wait until the main tank on your team has the enemy team occupied. Rammus, Shen with their taunts, Amumum's stun, Malphite and Alistar with their knock-up's. Then you pop rupture when they can't do anything about and enjoy the rest of that combo.

3.) AP Cho will know when to back off a team fight. Just because you're huge doesn't mean you can't be killed. Keep in mind you are a huge target, and pay attention to your health. You really want to try and keep your stacks, so dying constantly because you didn't realize you were Ashe's target doesn't help this.

4.) AP Cho will not be afraid to feast minions for his 6 stacks. I think this applies to all Cho players, actually. If you have no stacks and it's midgame, eat a ****ing minion. By the time you get in a team fight again, Feast will be up, especially if you have your CD items.

5.) AP Cho <3's solo lane. Something else that maybe can be applied to all Cho's, or anyone who can handle solo top. Solo lane, especially 2 v 1, is often great for out leveling and out farming your enemies. 9/10, in a 2 v 1 lane, your enemies will push to your tower, and you can sit there and lap up free gold and exp, with the tower protecting you. You probably won't get too many kills early games, but once they get impatient or your jungler comes in for a gank, you will probably have 6 stacks with feast and are a force to be reckoned with. Even in a 1 v 1 lane, you should be higher level than the enemy duo lane and jungle, with more gold and farm, which can only help you get kills in team fights later. Of course, don't be a **** to your team just to get solo lane. Please.

Essential Basics for Everyone, aka Ignore this if you know it Already:

Farming: You ever see those bumper stickers? The ones that say "Without Farms there is no Food?" This applies to League. Without a good Creep Score, you have no gold, no gold means no items, no items means no death machine Cho, and that means you are not going to make food out of the enemy team. Pay attention to your CS, kill jungle creeps when you have nothing else to do, grab Dragon with your team when you can.

Last Hitting: I hate it when people auto attack minions during lane phase. You push the lane right up to the tower, and give enemy laners free exp while you can't stop them. The concept of last hitting, therefore, is very important. For those who don't know,last hitting means only whack a minion when it's about to die. (push s when standing still to turn off auto attack.) When properly last hitting, it keeps the minion line more towards the middle of your side of the lane, and it means enemies have to leave their tower to try and get gold and experience. When they leave the tower, since you are a vastly superior player (hopefully) you will either kill them or make them back off. If they leave lane, that is when you start pushing on the tower.

Zone Control: This is something that is good to master. The basic is that each character has a zone, or where their champ's effective range is.(Note: Towers and Minions have a zone too) Things like brush, towers,ally minions, or akali's shroud, give you more zone. The more zone, the better. If you and your lane partner are properly last hitting, can use your zones to force enemy champs out of the range for experience, keeping them under-leveled and under-farmed, which makes them easier to kill.

Wards Save Lives: Have an extra 200 gold and heading back to lane? Grab a ward. Throw it in the river bush. The enemy jungler got Dragon behind your back? Put a ward there so your team can wreck his day. Those Teemo shrooms doing a number on your health? Grab the pink ward and pop all of them. Wards are awesome, so you should use them.

Adapt your Build: Just because you found this "super awesome build!!11!!1!" online does not mean you should always follow it 100% of the time. You have many different choices of items, a different play style, and you will face very many different teams. Don't be afraid to build a banshee's veil if you need it, or a thornmail if the AD carry is ruining your day. In the end you should be trying to not feed, supporting your team and knocking over towers, however you can.