Kog maw its a hybrid very low ranged champion which involves tons of damage as a carry.
Because of his verygood passive u will only need some life steal,buffs and attack speed because all of his spells deal magic damage except his ultimate which gives hybrid damage.

Take hybrid masteries and ad runes.
Berserker's graves for more attack speed.
Nashor's Tooth for mana regen,attack speed and ability power.
Phantom Dancer will give a bonus of critical strike chance,attack and movement speed.
The Black Cleaver for the buff attack speed and attack damage.
The Bloodthirster for the lifestell and attack damage.