-You are walking through a forest. Suddenly you see this monster, big as a tower- 10 times harder to kill-, spring out of the bushes! He screams, you fall to your knees in agony. He calmly leans over and devours you whole.
Cho'Gath is a super beastly tank. He can get ridiculous amounts of health, and can do minimum 800 true damage to any enemy. He can silence and stun any enemies that stand together and gains health for every kill he makes.
This guide focuses not so much on Cho's damage, but his survivability. With this build, I have personally tanked both nexus towers, 3 champions and two creep waves long enough for both towers and 2 of the champions to be destroyed. This build is very easy to use, and brings you one of the nastiest tanks in the game. Using these exact plans, I have double tower-dived at level 9 for a double kill.

Build 1
Pro-
Huge Survivability
Con-
Not as much damage as could be, but with Atma's Impaler endgame increases dramatically.

Build 2
Pro-
Lots of damage through AP
Still has some tanky capability

Con-
Not tanky enough to be the only tank on a team

Build 1, you are mostly looking for massive survivability and some CDR for more use of Feast.
I stack mostly health, with plenty of dodge, some armor and MR to minimize damage, making me almost impossible to kill.
By the end of the build, Cho' gets
-5k Health
-26% Dodge
-26% CDR
-50.15 H/S
-3 M/S
-229 Armor (70% DR from Physical)
-106 MR (51% DR from Magic)

*Note* -Cho'Gath has two weak points= Magic and %health stuff. End build, the damage from casters still really isn't much, but anyone with Madred's Bloodrazor and such is someone to be careful around.

Build 2, you focus on your raw damage, while still giving you enough survivability to not be a total pushover. You will end the game with
-3k Health
-2K Mana
-540 AP
-26 MRP (+15% after that)
-24% CDR

My masteries are very simple. They cover good survivability with a bit of CDR and MPEN from the Offense Tree. (MPEN because all of Cho's abilities but Feast are magic based.) While I take Ghost as a Summoner Spell, I don't use a mastery on it because I use it less for chasing and more for getting around. I also don't take the Ignite mastery because 10 AP really isn't much for Cho'.

Build 2
I go with a fairly typical caster build. It maximizes CDR and regens, while still giving you the magic penetration of the Offence Tree. I make sure to get the points in Insight because it helps your team a lot more. I also grab Blink of an Eye , because Flash has such a long CD.
*Note* You can go with the original Masteries build. It will defiantly make you more tanky. But the idea of the AP build is to be less tanky for the favor of more damage.

- Rupture is your main harass, so I level that as quickly as possible. It does good damage at level 5. That good damage becomes great damage if you get it fast, because champions don't have much health yet. It is also your main chasing tool, as it stuns them long enough for you to be in range of a Feast.
-Next most important is the Vorpal Spikes because they help with your attacks on players and creep waves. It is so wonderful to attack a creep and see your spikes hit the champion behind it.
-Lastly is the Feral Scream. I get the first level right after Rupture because it is very important to have a silence, but it's duration only increases 1 second over 5 levels and its mana cost increases when you don't really want it to, so I level it last.
-Leveling Feast is always top priority because it deals true damage, gets you bigger and honestly scares the **** out of melee enemies. If I am approaching a lane, I like to Feast on a jungle monster so that I get a stack of Feast, but the enemy champions don't know that it's on CD. If they think you can use it, they are afraid. If I have all 6 stacks of Feast , I sometimes don't wait for it to be a kill to cast it. If I damage them enough to push them out of lane, it is worth the cost. (usually)

Build 2
- I max out Rupture before anything else. It will be your main stun, damage dealer and chasing power. Having this as high as possible is important.
- Feral Scream is next, as it is your next best damage dealer, in addition to its silence.
- Feast is gained whenever possible, because True Damage is pure power.

My runes are a bit interesting. I pick them for both attack and defense.
-I pick Greater Mark of Attack Speed to improve last hit chances, and to have my spikes do more frontal damage overall.
-I pick Greater Glyph of Scaling Cooldown Reduction because Feast is true damage, the more you can use it the better. With this build you will end up at about 26% CDR, which means 44.4 second CD on Feast.
-I pick Greater Seal of Evasion because having a tank with 5k health AND 26.75% dodge chance is a very daunting task to kill. Dodge is great for trying to escape as well- they use a melee stun and you dodge it, you get away free.
-I pick Greater Quintessence of Evasion for the same reason as I pick Greater Seal of Evasion.
-I don't pick any of the other things because, really, they are insignificant in almost every case.


Build 2
- Greater Mark of Magic Penetration let your spells hit that much harder.
- Greater Glyph of Scaling Cooldown Reduction reduces your CD's so you can spam more and deal more damage.
- Greater Seal of Scaling Mana Regeneration increase your mana regen, which means you don't have to b after spamming.
- Greater Quintessence of Scaling Cooldown Reduction for the same reason I had Glyphs of Celerity.
- Greater Quintessence of Magic Penetration for the same reason as Marks of Insight.
*Note* I haven't bought Quints for AP Cho', so I don't actually know which is more effective at this time.

*Note* Almost all items are situational. Be smart when you buy things.
-Before leaving the starting zone, I buy a Regrowth Pendant. I get as many last hits as I can so I can buy the Philosopher's Stone ASAP. I continue to get as many last hits as possible till I have enough for the Heart of Gold. After that, last hitting becomes less important. I work with almost entirely pre-made teams and they know that I need to get most of the last hits till I have those. They are both solid tank items that build into good tank items, that also give gold.
-After that, I build a Ninja Tabi for the dodge, armor and speed. All great things for a tank.
-Next is Warmog's Armor. Why not Force of Nature? Because Warmog's needs time to build for full awesome. Why not more Warmog's? Because it is a very limited item. A build with all Warmog's is not very useful- if you have a billion health, the other team will just ignore you and kill your friends.
-Next is Force of Nature. You get MR and Regen, and some free movement increase!
-By now you have lots of health, so getting Atma's Impaler is your best move. Suddenly you are doing 100 extra damage per swing and can crit- scary stuff!
-I usually save a bit from the gold generators before upgrading, because I am almost indestructible at this point anyway. I build Randuin's Omen first, mostly because it is more expensive. Either item gives CDR and both Regens. The Omen slows opponents, while the Shurelya's Battlesong speeds your team up. The choice of which first is up to you.
*Situational Items*
- Guardian Angel- If you are getting the **** kicked out of you and need to survive better. Honestly if you need this, you need to learn Cho' better or get a better team. (or both)
- Leviathan- If you are raping hard and wont lose stacks.
-Other Boots- Situational boots are situational- all up to you.

Build 2
-I begin by buying a Sapphire Crystal. It gives you a little more spam early game and builds into one of your most important items rather quickly.
-I upgrade the Sapphire Crystal into Catalyst the Protector asap. The benefit is quite enormous, especially the boosts when leveling.
-I grab my Boots of Swiftness next. They are what lets AP Cho' get around so well and be where he needs to be in every fight. They also help you dodge skillshots which might push you out of lane.
-I then upgrade my Catalyst the Protector up to Rod of Ages. This should be done as soon as possible so the Rod has time to grow and actually be effective.
-Everything after that is situational. If they have a lot of AD champs, grab Zhonya's Hourglass first. Should they have lots of AP, get Abyssal Mask.
-After one or the other of those, get Rabadon's Deathcap. I don't like rushing Deathcap, as it is really expensive and it is easier to farm up with one or two other items first.
-Because of AP's lower survivability, I grab a Banshee's Veil. If you find that you are raping quite hard, I would switch for something like Void Staff.

These are really up to you, seeing as I used no masteries for any.
I pick Ghost for the move speed. IMO it is better than Exhaust because you can use Ghost to get around the map and you don't have to chase the same enemy. I pick it over Flash because you can get much farther with Ghost than with Flash and the CD is shorter anyway.
I pick Ignite because Cho'Gath really doesn't have the best finishing or chasing ability. If they are getting away and you can only be close enough for a single Feast, throw an Ignite and lol at their face as they die even though they thought they escaped.

In the AP build, I pick Flash -for better ult placement- and Clarity so you can spam your abilities with impunity.

-Early Game-
Most tanks are not very useful early game, Cho'Gath being no exception. Do harass with Rupture and Feral Scream, but worry most about last hitting the creeps so you can get gold. You want to be very careful, as you don't yet have the survivability that makes you awesome. Keep your mana in mind as you harass, you don't want to miss a chance to get a kill. At the same time, you want to be winnowing down their health, hopefully pushing them out of lane.
-Mid Game-
The transition from early game to mid game is not a hard line with almost any character. Once you start getting some survivability and you are working as a team on your ganks more than laning is your mid game. You are the initiator and the finisher. You stun with Rupture while your team destroys them. You make them useless with Feral Scream so they can't fight back. You make them dead with Feast.
*There is NO KS-ing with Cho'. Feast is Feast is Feast. You need the kills, end of story*
-Late Game-
This is where you are almost done with your build, but not quite. You are very hard to kill; in fact if you have two people with you- impossible. You should be trying to gank people on their team with streaks so you can finish your build as soon as possible. You can tank towers, no sweat and just need others to come power them down. Be careful of ganks at this point, because a death here to a 1v5 will set you back farther than you want to be.
-Full Build-
You are the ultimate unkillable force. Even with that said, don't be stupid. You can easily backdoor a tower, but don't stick around to go for then next or you can expect a 1v5 fight. (which you will probably be able to escape, actually.) You should be with your team, leading a push into your enemy's nexus FTW. You need to be trying to MAKE their high powered people attack you, to protect your team.
-Other stuff-
Usually, you will not be the chaser. If you find it to be the case, be careful with your placement of Rupture, it is your only tool to catch them.
Once you get Feast to full (6) stacks, always save it for players only. You will never again need to NOM a creep. Also- after 6 stacks, you don't NEED to actually kill with Feast. Using it to make someone b, *can* be good enough. (they are losing out on XP and gold, plus they now can't go gank your team for a while)

Build 2
You begin the game much the same as with tank Cho'.
Later, you will be dealing a lot more damage then with tank, but you will not be able to take the same damage you would with the tank build.
With AP Cho', you easily can be the chaser. With the base CDR of 24% (With the Quints of Celerity) you can spam your abilities and stun/silence them much more frequently. Plus Flash will let you jump in and Feast when they might not expect it.
You will not be able to survive nearly as well with AP. You should let your real tank initiate, especially if you didn't buy Banshee's Veil.

Cho'Gath is a beast, hands down. He is by far my favorite champion to play. There are many ways to play him, but with a well balanced team, this is the tank option that has worked best for me.

I have tried several AP builds and I find that the best ones still let you be quite tanky, so that you can blow stuff up while still being hard to kill.

Comments to improve would be wonderful!

BTW-First guide, so go easy on me. =P

*EDIT* Added all the little pictures. =)
*EDIT* Added an AP build that is actually GOOD at being hybrid tank/AP. I still prefer to play straight tank, but there are defiantly times when AP is quite useful.