Kog'Maw Runes explained
There's one set of runes for Kog'Maw on patch
12.12
that is the clear favorite for LoL players and pros alike. The best Kog'Maw Runes on the position 
Adc are Precision as the primary path and
Resolve as the secondary path with a pick rate of 46%. Besides runes you should take a look on Kog'Maw Build and if you are interested in pro matches take a look at Kog'Maw Probuilds.
Primary runes path on Kog'Maw
Precision improves attack and sustained damage on Kog'Maw troughout the match. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Lethal Tempo is the best keystone rune to run on
Kog'Maw. This keystone rune gives Kog'Maw a great head start in team fights and allows to put out more damage in that time window. Other primary runes on Kog'Maw are
Presence of Mind,
Legend: Bloodline and
Last Stand.
Presence of Mind helps Kog'Maw with the mana problems by restoring missing mana, if a champion is killed.
Legend: Bloodline gives life steal, which aids in winning damage trades and fights in the later stages of the match. With
Last Stand you deal more damage while you are at low health.
Secondary runes path on Kog'Maw
When it comes to your secondary path, most
Kog'Maw players prefer to go with
Resolve.
Resolve empowers the survivability of Kog'Maw and increases the tankiness. On the
Resolve path, the best runes for KogMaw are
Overgrowth and
Conditioning.
Overgrowth increases your maximum amount of health.
Conditioning powers up Kog'Maw tankiness after the laning phase.
FAQ for Kog'Maw Runes
Which is the best keystone rune on Kog'Maw?
The best keystone rune on Kog'Maw is
Lethal Tempo. This Kog'Maw runes setup is then referred as Kog'Maw Lethal Tempo.