As someone who plays Kog'Maw with above a 50% win rate with 430 more games than you in ranked SOLELY on Kog'Maw with the 9th most games play on Kog'Maw in NA I would like to point out that your mentality about Kog is the same mentality that was established 2 seasons ago. Items have changed, gameplay has changed, meta has changed. Any items that benefited Kog in season 5 have completely changed. BoRK gives only 6% max health bonus physical damage. Triforce doesn't help Kog get into mid-game, Guinsoo's is the perfect example of the concept of on-hit that doesn't scale and is UNDER EFFICIENT without at least 3 attack speed which is no longer possible for Kog.
Riot "buffed" his ult by reducing execute damage and increase poke damage. This actually nerfed AD Kog'Maw. They increased his AD ratio on ult but only applies to BONUS AD, so you aren't getting the max potential of his ult unless you build raw AD. Additionally, Kog'Maw's kit is an absolute mess. If you build lots of AD, his other 3 abilities that only scale with AP are useless. AP build is viable, but Riot promised to nerf AP Kog'Maw to hell if it became popular (which it is) because of how unhealthy it is.
So not only does Kog'Maw have terrible item choices, his kit is garbage too. I shouldn't be forced to play a different champion because Riot can't figure out what to do with him.
p.s. as an ADC Kog'Maw player, nothing is efficient for Kog'Maw right now. If on-hit works, you got lucky. If critical build works, you got lucky. If you got fed and won lane and the game, you got lucky. Guinsoo's requires 6 auto attacks to ramp it up which is easy with 2.5 attack speed. But that requires 2 AS items and Guinsoos. The only AS items provide crit which since you're not building crit (IE, ER) you're making that part of the item completely useless. This leaves 2 item slots for a defensive item, or BoRK (which is garbage), or Triforce which costs TOO much for no stats that scale. 12% max health as bonus magic damage is great, but if you take a 2k health, you only do 240 magic damage on hit. This is reduced by MR (suppose enemy has 80 MR), the damage is reduced to 133 magic damage on hit. BUT this is 133 every other hit. Do you see how inefficient it is? Or do I have to further explain myself because I can.
http://na.op.gg/summoner/userName=vorattasridon
http://www.lolskill.net/top/most-games?filterChampion=96&filterRealm=NA
ALSO, I put /s at the end of the post. I was being sarcastic to bring awareness to how shitty Kog'maw is currently.