Not Updated For Current Season
Just thought id share my fun way of playing chogath; its an ap cho because he's not a great tank and i just love mages! So consider him a tanky mage :)
Unlike other cho's, this one is very fast with: Ghost , Mercs (not swiftness cos u dont wanna get tied down by cc!), Zeal , FoN , and Dancer ! (This is almost 516 movement speed if u manage to get all the items! And this is also excluding the bonus effect of ghost!).
Now for his setup i like to use 9/21/0 for points into spell penetration and tankiness (you get all the mana regen u need from clarity and his passive - when killing creeps you gain health and mana). I also like to use Ghost and Clarity for summoner spells since ghost gives u a good escape/chase mechanism and clarity gives u mana for early laning phase. Other options are teleport and clarity or even flash.
For runes i like to use MP reds , health yellows Greater Seal of Vitality, cd blues , and mana regen quints . As for the skills, get 1 point in spikes and level up rupture and scream at the same time (since they are ur main damage spells apart from ur ult ).
Playstyle with an ap is hard to master, i recommend a lot of kiting, but basically u rupture to harass, then when you are ready you rupture behind them and move forward. This will cause them to flee backward into ur rupture, at which time you scream at them for silence, atk them a few times (with vorpal spikes left on) and finish them with ur all famous Feast! Now i also suggest that you become big before u decide to gank, at least 3 stacks big, and this can be achieved easily by feasting on minions or better still by feasting on the blue buff on the way to a gank :)
Anyway have fun trying this build out! GL HF ALL! :)