My usual is:

Mana Manipulator ---> Soul Shroud (I get mad when I don't have enough to feast and the champion in front of me is really asking for it)

Ninja Tabi

Aegis of the Legion

Guardian Angel - by GA...game is usually over...but if it's not...

Frozen Heart - It's really over now...

???

I was thinking frozen heart as one of my first items or replacements for mana manipulator

I believe my core build is:

Doran's Shield
Banshee's Veil
Guardian Angel
Mercury Treads

And then I just split off into whatever I need more between MR and Armor. Frozen Heart or Thornmail for melees and Abyssal Scepter / Force of Nature for casters.

I'm not much of an early ability spammer. I usually end up 2v1 whenever I'm Cho so I max feast, then vorpal spikes, then silence, then launch and I usually dont have mana issues early game.

So your core build is all armor and HP with no MR?

Personally i don't have tank builds, i just build what i need to tank the enemies and spend the rest of it on AP/AD.

The only real Cho Gath build is an AP Cho Gath build. Theres no incentive to attack him if hes simply an unkillable meat shield.

I start with Null-Magic Mantle or Cloth Armor then work for GA and Ninja Tabi/Mercury Treads, prioritizing depending on the enemy's team composition and how long I can stay in lane. Best case I can stay long enough to get GA + Boots 1, if I'm pushed out of lane sooner I get upgraded boots + as much of GA as I can.

Then I usually get Heart of Gold, Aegis, BV and upgrade the HoG into Randuin's

This is pretty similar to how I build Nasus, except with Nasus, Aegis and GA are switched around.

Go AP/AS, you'll be suprised.

Rageblade, Nashor's, hell even get a Madreds for lulz.

Try a Doran's Ring for starters and a health pot, this should be pretty good for keeping your mana up early game. First real item you pick up get a Rod of Ages, this will get you Health, Mana, and some decent AP to work with. From here on build what you need to, if people aren't doing much damage to you then feel free to resist buying tanky items and get some damage (aka some AP).

Try it sometime, you will be amazing with the damage his Rupture can do (if you focus it) with a little AP, not to mention +AP will make his Ult deal more damage.

I normaly start with a Regrowth pendant and a mana pot rellying on runes and last hits for mana regen. Grabbing a second regrowth, boots and when I can a Chain vest. Then from there I go FoN or Frozen heart depending on the team your against