Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build is still under construction. Right now it's primarily a way for me to keep track of the changes I'm making to my build, and constructive criticism is welcome. My focus (in order of precedence) is Magic penetration, AP, and health.
THIS IS NOT A TANK BUILD
You will not have the durability of tankier builds, but you will have much more magic penetration, and comparable AP to the AP/off tank builds along with 3800 health. The strategy with this build is to harass from the sidelines and to dart in quickly to pick off runners or enemy champions who are low on health with your ultimate.
If you're looking for something with more durability I recommend this off tank build or this tank build.
Pros / Cons
High magic penetration
Reasonably high AP
Awesome harassment capabilities if you can master rupture
Low armor and magic resist
A little squishier than other Cho cho builds
Difficulty running away from Ganks.
I focus primarily on magic penetration and ability power with my runes, but I save my seal slots for cool-down reduction as most of my damage in team fights comes from my abilities rather than my auto-attacks.
I use exhaust and Ghost. These are useful for catching runners, which is where most of your kills will come from, and Cho'gath is quite slow.
Exhaust slows down your enemy (which combines well with the Rylai's and Rupture) allowing you to catch up and devour them.
Ghost gives you the extra burst of speed you need to catch up to your next snack. I also find it more useful for escaping ganks than flash unless I am playing very close to towers.
Flash is also feasible, though it changes your strategy somewhat. If you go with this spell you're going to want to take the point out of haste and put it in good hands, and then shift one of your armor points in the defensive tree to In the Blink of an Eye. Offensively, you'll be wearing enemies down with your harassment capabilities and then flashing to enemies with low health to finish them off with your ulty. Defensively you need to be more careful of how you use Flash to escape ganks than you do with Ghost, preferably you will go through at least one wall, but I find that the enemy team is usually prepared for this, and Cho' is slow enough that they often catch him anyway, and given that this build is vulnerable to ganks, I usually prefer to just stick with Ghost.
I go with a 9-6-15 build. Offensively, I focus on AP and cooldown reduction. You could theoretically put some of the defensive ranks into Offensive mastery, but I like the durability that the points in armor and MR give you early game. The utility masteries are focused on mana regeneration, gold gathering (this is an expensive build) and speed.
Pretty straightforward. I put the Rylai's and Warmogs in there to give this build a little more survivability so you can run into a team fight and pick someone off then get out again without dying. I've seen people put archangels into Cho, but I never run out of mana in mid-late game teamfights and between your masteries and Cho's passive you should be able to regenerate mana like a speed demon while laning.
The last item slot is the most dependent on the opposing team. I put frozen heart as the last item so I can use the debuff to detect ganks. If the opposing team is primarily magic damage based then I usually substitute Force of nature or Abyssal scepter. You can also place Banshee's veil here if you need to combat crowd control. In games where I find that I don't need a defensive boost I'll put Rabadon's Deathcap here instead.
The first rule of this build is WATCH FOR GANKS. You have enough health to survive for a little while, but you're quite vulnerable to any well prepared team. If you maintain map awareness and use the frozen heart debuff to check the bushes then this problem should be fairly minimal.
When laning you want to harass with Q and W. Rupture is an extremely effective harassment tool IF YOU LEARN HOW TO USE IT WELL. It has an extremely long lag time, and so you have to predict your enemy's movements in order to hit with it. This is a skill that takes time to develop, but two of the techniques I find most helpful are placing the outer edge on the opposing champion so they have to run all the way across the AOE in order to get away (this requires knowing what direction they'll run) and waiting till they've committed to some other action (such as attacking your tower) before hitting Q. An added bonus to this strategy is that harassing while your enemy is in the middle of their creep wave can result in a buttload of last hits when you combine this tactic with Vorpal Spikes.
If you have good map awareness then you can be an extremely effective ganker. Knock the enemy into the air with Q, then hit them with W as they land. This should wear them down enough that you can hit them with a few autoattacks to lower their health, and then finish with your ulty. If they run you may need to use exhaust. If you're ganking with a teammate then you need to use your awesome slow powers to keep the enemy from getting away while your teammates finish them off. Though you should still devour them if you can.
During team fights you should hang around the edges of the melee harassing with Q and W. Don't strike with your ulty unless you see that the opposing champion is low on health, because you're going to want to save it for runners. If you have full stacks, then I usually try to hit the enemy's DPS champion so my teammates can finish him off (Gangplank is a tasty rum-soaked treat). As always, maintain map awareness and try to stay a little bit behind your team, otherwise you're going to get focused a LOT.
Farm farm farm. Cho'Gath has a green thumb when it comes to eating creeps, and you need to use it to your advantage. When harassing with Q, try to wait until the enemy is in the middle of a creep wave. That way you'll have a bunch of low health creeps coming after you which you can then last hit using the AoE on your E toggle.
Once you get your Ulty you should be using it to last hit the big creeps unless you know a team fight is imminent, or you're already at full stacks. The cooldown is quite low for an ultimate, and this strategy will increase your already impressive health counter, which gives you more survivability in a relatively squishy build.
Once you get to a little higher level you should be able to one hit the wraiths with Q, so hit them and Blue whenever you pass by (assuming your jungler doesn't mind).