Cho'Gath was the first champ I bought and where I have earned most of my xp/ip. The stats on this build look a little mediocre, but looks are VERY deceiving in this case.

DISCLAIMER: This is not your typical Cho'Gath build! I've played him as a tank many times and mixed it up a bunch with builds, but this is the one that I've settled on.

Warning: You are squishy for a little while with this build and will have to play that way. It will be worth it in the end. I tell you this from experience as I often have more kills and fewer deaths than our team's carries (plus a decent amount of assists).

PLEASE TRY A FEW TIMES BEFORE RATING
NOTES:
-For this build to work well for you, you need to enjoy harassing and learn how to anticipate where your opponents are going to move.
-Please make sure your team isn't thinking you're going to be the tank if you're using this build! I usually wait until we have a tank to select cho.
-This build is primarily made for summoner's rift, but is viable in the twisted treeline. Just swap out ghost for teleport in both the mastery and summoner spell.

I felt to put this at the top because I, myself, will often just look at items, see if there's anything interesting by skimming and then play.

Rupture: Feigning works well with this. It'll keep them back. Also, try to lead their champs rather than putting rupture right on them. If they're running away, try to get close enough that you can lead them with this and easily catch up. Also, staying in brush when casting this helps due to the fact they can't see the character animation.

Feral Scream: This has more range than it shows. I have finished people off with this many many times when they weren't in the blue part that shows the range of the spell. Test it out because it's hard to explain, but it works.

Early game:

Try to select the lane that is closest to your team's blue golem, unless your team has a jungler, in which case go for the lane closest to the enemy's. I will usually get in the brush on my side and try to stay there unless last-hitting. During laning, your goal is to keep them away from last hitting your creeps while you last hit theirs. I usually wait until level 3 to harass because level two rupture hits a lot harder and you want your mana for that, but that's why you have a mana pot. Use rupture when you're confident you can land it (doing it from brush helps because they can't see the animation of you casting it). Try and last hit to keep your mana and health up. You can be fairly aggressive, but, obviously, don't over-extend and get ganked. I usually head back when I have enough for malady, or as soon as I'm level 6, whichever comes first. At 6, you need to go get the golem buff. NOTE: cho'gath's feast currently has an interesting feature (Please don't ever nurf it riot!). Once blue golem is at <1000 hp your level 1 feast will eat him! Get blue and go harass in your lane a lot! The cdr should help you feast more and get kills and/or get bigger and tougher to kill.

Mid-Game:
I usually hold my lane and get plenty of kills there. GA and abyssal are usually purchased during this time. Harass and when the opportunity is there flash in and eat them. PAY ATTENTION TO THEIR HP! Feast does true damage 300/550/800. I love the 800 hp finisher.

Late-Game:
I don't really know what to say here. Don't charge in and get killed, but don't be afraid. Use rupture to hurt and slow those wonderful melee champs, and try to use feral scream to silence their team (particularly useful on fiddlestick's drain might I add). Eat people every chance you get so that 5v5 quickly turns to 4v5 and they start to run. Use rupture to catch them by throwing it in front of their champ (people run into it a lot).

I use these runes because it makes rupture hit very hard early game and I enjoy making others blue-pill.

Fairly standard 9/0/21 for masteries.

Malady:
Why attack speed (Malady) for Cho?
-I get asked this in many of the games I play. The main reason I use malady is the fact it lowers magic resistance. This helps me get blue easily around level 6 for lots of harassing and cdr for feast, and also makes vorpal spikes hit harder. Also, getting amp tome plus your runes will make rupture hit for a little over 120 at level 1.

Sorcerer's Boots: Magic pen is helpful in this build. I tend to spam spells a lot and do a lot of harassing.

GA: Your stacks are what keep you alive/intimidating/etc. This helps protect them. If you're up against a lot of ad get chain mail first, ap get the magic res. I usually get what I can afford between magic res and cloth armor if it's mixed.

Abyssal Scepter: I usually get the ap first so my spells hit harder, but if you're getting beat up by ap (teemo and your little mushrooms of death, I'm looking at you) get the magic res.

Frozen Mallet:
Why get frozen mallet?
-This gets asked often. It gives you the hp that you will be needing to adequately intimidate people and stay in team fights. Also, it slows them down so you can pound on them more.

Frozen Heart: I get this end game if it lasts that long. In conjunction with the mallet, if I get hit, they're slowed, if I hit, they're slowed. Moral of the story, they're slowed...

I find that this combination works well for me. I would say the first 3 items are your core and if you want to mix it up from there to fit your style, great!

I'm not sure if I put the 2nd level of Feast at the right level. This guide assumes that you are hitting decently with rupture.... If not, a: practice makes perfect; b: get feral scream maxed so you get guaranteed hits. Long story short:
1. Max rupture asap
2. Vorpal spikes at level 2 so you can last hit/farm easier and replenish that life/mana using your passive. It is good to have, but your caster skills are more important most of the game.
3. Max feral scream asap after rupture
4. Get Feast as often as possible.

I'm pretty much set on flash/teleport. I usually save flash to get out of a bind, but if the opportunity is there, I'll often flash to eat someone (non-team fight). Teleport is one of my favorites so I can get back in a lane and get xp/gold soon after shopping. As with the game, get what spells fit you. I just find these two work best for me in this build.

With this build, you will probably not be the one destroying all of the towers, but champs will fear you. When played properly, you will usually get 10+ kills, <5 deaths (I'm usually around 2 due to diving and my aggressive play style), and lots of assists. Your job is to slow the other team down, eat people who think that 780 hp means stay, and to silence and rupture people like crazy. Like I said, try it before rating it. I enjoy playing cho'gath using this build because people rarely expect a cho'gath that isn't a tank. From what I've seen, this is a fairly unorthodox cho'gath build. Hopefully it'll work for you.

NOTE: The stats of my last game were 20/11/15 and it was a rough, close game (80% of my deaths were early game from a xin/jarvin combo...) Late game you are a beast though.