I've been playing for a good time, since before the rework and all. I have a really different view on how to play it. First of all, I'm actually nearly S2 from B4 just playing Kog (have like a 70% winrate).
The core is alright, Guinsoo, Bork and runaan (AS Boots ofc). But for the 2 last items, I hardly ever buy Last Whisper. I have enough % dmg to not need it, BUT, if they have at least 3 guys full armor (think of a Rammus/Malphite/Braum or even Nasus, Mundo), I'll take it. Also, I never used Muramana or Essence Reaver (not even infinity). In general, I build 2 defensive item Sterak/Frozen Mallet, and after, either Banshee or Randuin. It's really useful, so you can be a bit "on your own" sometimes, like, you survive a Zed's ulti and even kill him (an assasin). If you really rely on your support you might buy more offensive items. Of course as a Kog'maw you're supposed to have a good positioning but if their team is composed by a Malph Top, Diana jg, Zed mid and a Thresh, they could reach to you whenever they want.
But your build itches my curiosity. I'll give it a try :)

Please remove tear from the build. A champion that is already so weak early game does not want more scaling.

Thanks for the advice ^_^ yeah I clearly need to put some more work into those unpolished sections of my guide, I honestly wasn't expecting Kog to get this popular SO FAST, so I was unprepared for the extra views and viability xD
Thanks!

It may be an idea to update the lane matchup, and include a chapter about viable supports for Kogmaw.

As an example a matchup where you have "AP" supports while the enemy carrier has a support than can dive without any consequence (Leo / Braum), will unavoidably result in being perma stuck on your tower afk farming, and it doesn't really matter what enemy adc you face then.

Outside of the above, great guide and +1

You are right there, I clearly forgot to change that, a few patches ago Kog's %hp damage scaled with his AD, then riot traded that scaling in for the 2% base amount. The Bloodthirster is still a decent item, but it's still a situational item to be used when you have little to no faith in your team. I still dislike the Hextech Gunblade since it really has very little scaling with Kog'Maw. Thanks for pointing that out :D

And since that is the case, does it change your thinking on items with AP in them such as gunblade and/or the effectiveness of bloodthirster PD / Mer Scimitar PD combo?

I don't understand, You say the Bloodthirster will almost give you a full extra percentage on your w. But the W say's it gives percentage based off of AP.

Hey, I'm glad you like my guide, thanks for the response :D
You are right about the AS passive, I think I got that switched over, so the leveling order should reflect the changes.
As far as boots, that is a matter of personal preference, I like being super powerful glass cannon, but the boots won't hurt your late game by too much and if you are more comfortable on them, then that is definitely what you should do. Skipping the boots makes you more vulnerable as well, but higher damage, just a tradeoff, neither way is wrong.
On the runes, no they are the least cost effective part of the rune build so go ahead and skip them if you are short on IP, but if you ever decide to finish out your page they are what i would suggest.
Hope you have luck with Kog'Maw (and the other champs too xP)!

Hi ! Thanks a lot for your guide !
I hadn't played Kog since his rework, so I was looking for a good guide for him, and this clearly is.
Here are my thoughts tho :
-Since 6.3, AS passive is on the W, not the Q. So I guess the skill order is R>W>Q>E now.
-I just can't play without boots. They are so useful to dodge skillshots, and sometimes you can catch someone, or escape. I may sell them in very late game, but I still buy those early on.
-Are blue AS runes really necessary ? Dont have that much PI and it doesn't seems like a worthly investment...
Totally agree with everything else ! :) Once again, thanks for the guide !

I do ult a lot, if you are very conservative with your mana and prefer not to ult often, the Hurricane is definitely the superior item. The reason I ult a lot is to keep people from getting away, it is only really an execute at this point, and is not to be used as poke, so you don't spam THAT much. The Hurricane also lacks scaling with Lulu or Guinsoo's, which is another reason I do not use it. The Muramana is obviously not just used for the mana though, as the active is extremely string on Kog'Maw and allows him to outright drain any enemy champion even faster, though it requires some micromanagement. Thanks for the response though :)

like the guide well enough, but you must use ult a LOT if you think tear is necessary. Because of how the ult works now compared to pre changes, the ult does next to nothing unless theyre below 25% hp. I would swap it for hurricane, because the extra 2 bolts now apply on hit effect (which is the purpose of the new build) to their full effect, meaning you can melt more than one person with good positioning. dont know how strong crits are with the new w too...