Let me say this now. You must forget about being a tank. Cho'Gath will never be palyed as a tank by you again if you follow this build, he will be to you a hybrid DPS. If you refuse to believe this then go and find another guide.
For those of you who stay this build is set to play Cho'Gath in a very agressive style, allowing for strong lane domination from early in the game and heavy DPS late game.
Also as a side note I normally play 3v3 but on the other hand I have tested this build 5v5 it still works.

Pros

-Very good farming ability
-Strong DPS both through abilities and auto-attacks
-Strong lane sustaining with passive
-Can jungle while lane is pushed up

Cons

-No blinks (You can substitute Flash for Exhaust/Ghost to fix this although I recommend keeping the summoners above)

For my runes I prefer
Magic Penetration Marks
Attack Speed Seals
Flat Ability Power Glyphs
Movement Speed Quints

The M. Penetration is obvious for the amount of spell damage you deal with your Rupture, Feral Scream, and Vorpal Spikes.

Attack Speed Seals to increase the DPS with your Vorpal Spikes early game

AP Glyphs for all of your abilities

Finally Movespeed Quints because of the serious movement advantage you can have at level 1

For my masteries I go 9-0-21

9 in offense for the extra 15% magic penetration

21 in Utility to get the Exp bonus, the Movespeed increase, and increased CD on both summoners.

Your core build is the Berserkers Greaves, Guinsoos Rageblade and Starks Fervor.

-Berserker's Greaves over other boots why?
this build does alot of damage based on your Vorpal Spikes, thus extra attack speed really does help.
My exception to this is only for Mercury Treads, if needed for a CC heavy team.
-Guinsoo's Rageblade is huge in this build. it quickly lets you get a nice amount of attack speed while giving a large amount of AP, not to mention the bit of Attack Damage you get from it.
-Starks Fervor is again huge, by level 6-8 with this item you already heal at least about 25-40 HP per attack, also it buffs your team and gives even more attack speed for you to work with.

Afterwards it is up to you, adapt to the other team with your own mix of AP, Magic Resistance, HP, Armor, and AS.

My recommendations are:

-Rylais Crystal Scepter, Gives you a constant slow for you auto attacks, your Rupture, your Feral Scream, and your Feast.

-Abyssal Scepter, Lowers nearby enemies Magic Resistance while boosting your AP, not to mention some Magic resistance for yourself, whats not to love?

-Warmogs Armor, with your extreme farming ability this should give you massive HP bonuses.

-Bloodthirster, like your Warmogs it should get farmed fast, giving you plenty of lifesteal and damage.

-Atmas Impaler, Good paired with your Warmogs or Rylais for the damage.

-Zhonya's Hourglass, a good bit of armor plus plenty of AP, also the active for Stasis can help if you need to wait out your cooldowns.

-Nashors Tooth, personally not one of my usual items but it is very good. Reduces your cooldowns, increases your attack speed, and gives you AP. My only problem is it doesn't help with increasing your general durability, no Magic Resist, no Armor, no HP

Focusing your E, Vorpal Spikes, is very important for the major damage it deals but still you need to start with your Q, Rupture, for the CC and overall damage.

Get one point in your W, Feral Scream, at level 4 for the silence and put points in Feast whenever possible.

Personally I prefer using Ghost and Exhaust.

-Ghost, for easy chasing or if needed running away

-Exhaust, for those pesky little AD carrys, also to lessen damage of any type or just for the slow.

Other good choices would be:

-Flash, for the blink of course
-Teleport, on 5v5 can be a real big help

So in all, this build is a very agressive build. Lots of damage from start to finish.

Also this is my first guide so if there are any criticisms (which I am sure there will be) please let me know, I would like to improve on guides in the future.

Have fun with my hybrid Cho'Gath build :)
~Blind