Cho'Gath is not one of the best tops, but has a lot of potential. Its R let's you build up a lot of health infinitely (actual 255 max stacks).

All of its abilities have some kind of CC. Its Q (Rupture) makes the enemies airborne and slows them, W (Feral Scream) silences all enemies in a cone, and its E (Vorpal Strikes) slows them again. All Cho'Gath abilities (except R) are AOE and that makes Cho one of the best teamfighters: slow, stun, slow, eat.

Cho'Gath is good at teamfights but also good at duels thanks to its ult, but it suffers a lot to some top laners. When the R is gone a lot of 1v1 potential is lost. Its passive gives him a lot of sustain, well small but continous sustain, and therefore it can stay longer at top.

"Duelist" Style:
Not the best duelist, and actually no that good, but if you manage to lower the enemies' health you can easily kill it. Staying at tower is a good choice due to Q, make the enemy airborne, let the tower hit it, and end it with a Q.

Support Style:
This is the main style to play Cho'Gath, at least for me. Your role is not to be the player with best K/D/A at the match, but actively helping your team. Cho'Gath R is a better version of Smite, it's pretty easy to steal drakes or Baron when you are over 5k health and have an ability that deals 1,5k TRUE damage to monsters and minions. Whem teamfighting your objective is to take all enemy damage (only damage that would hit your allies, otherwise you are soaking damage for no reason). My favourite strat is to pop up Stone Gargoyle and tank all incoming damage. This let's your ADC burst without being at peril.

Flash: Must have. Best summoner spell currently, and make you able to dash and R your enemies into oblivion.

Teleport: Top laners friend. Cho'Gath is a slow chunky void creature, this allows you to be faster at top or help your team dealing with drakes, which you also want because STACKS.

Ignite: Solid pick. Can cancel out some of the enemy healing. Also makes sinergy with your true damage ult.

Rupture: Skill "shot" (I don't know the exact terminology for this type of abilities). Mid-difficult to hit. Lots of magic damage, one second of airborne to all enemies that happened to be there, and 60% slow once they fall.

Feral Scream: Worst ability of Cho'Gath set. Uses a lot of mana, the AOE is small, silence is pretty short, and takes a year to cast for some reason. Makes decent magic damage.

Vorpal Spikes: Don't underestimate the potential of this ability. Makes your next 3 basic attacks more powerful (not a lot), but deals a plus of 3% of Cho'Gath health that scales with Feast Stacks. Now you have a total of 9% Cho'Gath health dealt to all enemies. Did I mention that the size of the ability scales with Stacks? Because it does.

Feast: One of the funnier, most satisfying, and best ults at League Of Legends. A point and click ability that makes a decent base true damage that gets stronger with Cho'Gath total health and AP. This is a monster of ability that grants you 160 max health every time you kill an enemy champion with it.

Cho'Gath is that champion that you play because is funny to one-shot the enemy ADC and don't get lower than 50% of your max health. But seriously, maybe not a top laner with great presence but is a good drake stealer (and can solo it) and will soak damage for your team. Like it or not you are a support just by existing, taking damage and getting does sweet STACKS only makes you stronger and CHUNKIER.