Kog'Maw Runes explained
There's one set of runes for Kog'Maw on patch
11.9
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
Inspiration as the secondary path with a pick rate of 38%. 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. If you want to discuss
Kog'Maw then you should visit the champion subreddit.
Primary 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
Hail of Blades is the best keystone rune to run on
Kog'Maw. This keystone rune helps you to win short damage trades in your lane by increasing your attackspeed at the beginning of a fight. This is usually followed up by taking
Presence of Mind,
Legend: Bloodline and
Coup de Grace.
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. Kog'Maw deals with
Coup de Grace more damage to targets with low health.
Secondary path on Kog'Maw
When it comes to your secondary path, most
Kog'Maw players prefer to go with
Inspiration.
Inspiration opens creative opportunities on Kog'Maw troughout the match. On the
Inspiration path, the best runes for KogMaw are
Biscuit Delivery and
Cosmic Insight.
Biscuit Delivery is a great rune for laning as it gives additional health and mana, which can help in more aggressive lanes.
Cosmic Insight grants summoner spell and item haste.