I tried following the top rated cho'gath build but I came to the conclusion that having these items would be ever better. This has worked out greatly to me and many friends. I'm finally willing to share the build here, on mobafire!

I go for magic penetration marks, they really help the damage a lot. Other marks you could use are health per level or health marks.

I get armor seals, for early armor increase.

I get Magic Resistance per level glyphs 'cus I've come to the conclusion that having low magic res. isn't that bad early game, but having more late game does miracles.

I go for health quintessences, just to grab the +78 health to begin with. It's a lot early game.

I take 9 points in Offense, getting the bonus on exhaust, some AP, CDR and magic pen.

I take 21 points in Defense, seeing that Cho'Gath is a hardcore tank, having defense on masteries is only natural.
I get 3 points in increased mag. res. to make up for early game mag. res. lack, just in case. health per level and health to start with are good for cho, seeing that he prioritizes on health.
3 points in health regen. increase so I can solo the lane better.
2 points in movement speed for better get away or chasing.
3 points in CDR/lvl for faster skill spamming naturally.
3 points in reduced damage taken which gets me 1.5% reduced damage. It can be a lot.
and finally 1 point in juggernaut for more health and tenacity.

I start off with doran's ring for the mana regen and early AP.
I rush the mercury's treads to avoid incoming cc from ganks.
I get rod of ages fairly early to get the bonus health, mana and ap asap.
I go for Guardian Angel after that to protect my ulti stacks.
Then I get warmog's armor to stack even more demonic amounts of health!
I either get frozen heart + force of nature if I want to be really tanky and need both, or frozen heart + rabadon's / Force of Nature + rabadon's if they're lacking ad / ap or have a dominating champ (for example akali raping butts) I get force of nature and rabadon's to avoid taking her damage and dealing a lot of it myself.

I take Q on lvl 1 for maybe a leash on blue or getting away / saving a team mate if the blue is ganked and we can not keep it. I max it last, seeing that it is fairly high on mana cost and it isn't as good as the other skills on early/mid game.
I take E on lvl 2 to start farming better.
I get W at lvl 3 and max it first for the high damage and silence.
Naturally I try to keep my ulti stacks safe, leveling the ulti on levels 6, 11, 16.

The best way to combo your skills is to Q to throw them in the air, W to deal massive damage and silence the enemies, and keeping E on at all times to deal max damage. When they're low enough, hit R and kill them with it. Your ultimate deals true damage, ignoring all magic resistance and raping anyone.

Some people might get Clarity + Teleport to solo top, but I prefer getting exhaust. Just last game, They dived me on our turret. Sure I ended up dead seeing that it was pretty early in the game, but I exhausted the other enemy, Knocking the other one up in the air. I ended up killing them both (DOUBLE KILL!). Well worth it. Then I respawn, after purchasing the items I can get, I teleport back to farm more unless I want to save the teleport for a difficult situation.

Cho'gath in my opinion is the best possible solo top champion. Using this build, playing good, guarantees your victory.