AP Marksman Kog
Since AP Kog is making a comeback, I'd like to talk about a lesser used (but strong) build - AP Marksmen Kog. It's very similar to normal AP Kog, the only difference being that you build Nashor's and Lich Bane; the advantage is that it gives you VERY strong sustained damage, and because of Kog's low cooldowns he can proc Spellblade on cooldown. If you can safely auto attack in fights, this build is definitely the highest damage option for Kog. The basic build:
and final item
or 
Since Kog has low AP ratios (except for his E, which has a respectable .7 ratio), Magic Pen and spell- effect/ on- hit items are optimal for his damage.