Kog'Maw Runes explained
There's one set of runes for Kog'Maw on patch
11.15
that is the clear favorite for LoL players and pros alike. The best Kog'Maw Runes on the position 
Adc are Domination as the primary path and
Inspiration as the secondary path with a pick rate of 43%. 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
Domination improves burst damage and assassination tools on Kog'Maw. 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
Taste of Blood,
Eyeball Collection and
Ravenous Hunter.
Taste of Blood heals Kog'Maw as soon as you damage an opposing champion.
Eyeball Collection is next up. This grants the user additional damage with every killing they score.
Ravenous Hunter is the final rune of the
Domination path. This rune gives some survivability during short fights and damage trades during the laning phase. The additional health is nothing to scoff at and can mean the difference between winning or losing a battle.
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.