Across 665 ranked games on patch 16.21 in Master and above, Kog'Maw mains win 53.7% of their games in the bot lane.
Their most common build appears in 31.3% of those games and wins 57.2% of its 208 games.
It starts 
two Health Potions and 
Doran's Bow, completes 
Guinsoo's Rageblade, 
Navori Flickerblade, 
Terminus and 
Jak'Sho, The Protean in that order, and takes 
Berserker's Greaves.
Three other builds are played often enough in the bot lane to be measured on their own.
In the bot lane, Kog'Maw mains fare best against 
Ezreal (won 73.3% of 15 games), 
Jinx (won 66.7% of 21 games) and 
Kai'Sa (won 66.7% of 18 games).
Their hardest matchups, the champions that counter Kog'Maw, are 
Caitlyn (won 21.4% of 14 games), 
Yunara (won 38.5% of 13 games) and 
Jhin (won 51.3% of 39 games).