Cho'gath is a a very effective, and mostly feared champion in late game matches, due to his size, HP, and AP. With a perfect build, Cho'gath will be an unstopabble killing and eating machine. Good CC makes him good in hunt and teamfights. He can deal tons of damage with his passive Vorpal Spikes, because it gives you bonus damage from the stack, + bonus magic damage scaled by your AP.

As I always do, my starting is a Doran's Ring, 1 HP and 1 MP potion. Only use the potions in necessary, if you can't get HP and MP from last hitting with your passive Carnivore.

My next item is the Rod of Ages. It gives you good balance of HP, Mana, and AP.

The second item, is the Rylai's Crystal Scepter. Good HP and AP balance, and also works excellent with your abbilities.

The other 4 items are AP and tank, that makes Cho'Gath bigger and bigger...And more powerful...

The last item, is the Warmog's Armor, or if the enemy can't really kill you, then a Deathfire Grasp.

I always max my Q Rupture first, so I can take off enemys, and enemy minions more faster, and more effective. After that, I max my W Feral Scream. Of course, lvl 6-11-16 stack on ultimate Feast. E Vorpal Spikes is your last abbility that you need to stack. It will be effective at late game to get farms very quickly.

As Runes, AP and Tankness are balanced at me. First of all, 3 Greater Quintessence of AP, for starting AP, 9 Greater Seal of Armor, 9 Greater Gylph of Scaling Magic Resist, and 9 Greater Mark of Hybrid penetration, for the passive melee strikes, and skill using.

If you see your teammate is about to being hunted, try to use your Rupture behind him/her, so they will stop following you, this makes some time for your teammate to go away.

In teamfights, try to silence most of the enemy's with your Feral Scream, then Rupture the ADC, or cast it on enemys again.

In farming, I usually last hit minions. When geting more MP, and MP regen, I'm using my Rupture and Feral Scream to farm. If no enemy champion there to eat'em up, try to use your ultimate on bigger minions.

Pros

Good balance of AP and Tank
Hard to kill at late game
Effective, in hard push (Tanking)
Ultimate is perfect Execute skill
Good brush checking with Q

Cons:

Haven't got lots of early game advantages
Usually focused first in early game
Needs practice with Q
Having hard time not being seen when full at ultimate stacks

I'm glad that you have read this, this was my first guide, I hope I helped you with this, feel free to write comments about your opinion.

Thank you again so much for reading this!

(If theres any spelling problem, please, write about it in the comment sections)