This guide is not mine, its author, xhakami, wrote it on reddit. I just found it so funny that I decided to share it here. I leave you the link of the post in case you liked the build.
https://www.reddit.com/r/KogMawMains/comments/1bhfr3d/phase_rush_ap_hybrid_onhit_tank_kog_maw_guide/

Now why would I do this you ask In the following Guide I will explain my reasoning for this very fun and statisfying build.

For the lazy ones
Tldr:

Kog maw has more than enoguh dmg, but lacks mobility (only has e as escape which is only a slow...) and is squishy without enchanter, and thus explodes. IF you want to play games and not be reliant ont he off chance your support picks an enchanter:

Phase rush for mobility

early ap for wave Clear

on-hit because powerhouse lategame scaling

Tank because you it allows you to actually enjoy the game, and survive initial burst.

you can then finally live out the dream of kiting. It feels great.

Also you can actually just Statcheck most other adcs AND especially assasins 1v1 after Item + boots

also if you still hav ean enchanter you are now unkillable, as shields and shields plus resistance items complement each other

Your first item + boots always beats enemy adcs first item + boots(and even + half item probably)

Kog Maw was never a champ that lacked damage, he has two jarring issues though, he is squishy and he has absolutely no mobility. Something that an enchanter could maybe remedy. Now in a solo q environment that doesn't always happen, and if you so happen to want to just go otp kog maw style, that may be a problem. Cue to my build. I will list each section and explain exactly why i took these choices.

Phase Rush solves Kog maw's first issue. Mobility. With this rune you always have a rune that allows you to make a speedy escape, OR make sure to extend your trade in bot lane with an E auto, auto. You get to decide when the trade ends or not. It also allows you to get out of sticky situations, where you would normally be asittin duck, in a gank for example.
YES, you loose dps for not going lethal, but i can count on one hand the amount of games I lost early game, before getting to live out the lethal tempo guinsoos dream. Also because you can chase after your enemies you "gain dps" timers.

Mana Flow (the only really option unless you want to go fancier, look at the end for a special surprise version of the build with nimbus cloak)

Celerity, ok tbh, you COULD take Transcendence for the uptime for w and probably be better off, but well if I am going to go the speed route I may as well.

Scorch, VERY nice early game rune, and helps you scale safely into mid/late because of its pressure.


Side Runes:

Because of later Item choices free boots is the superior rune, yes I know boots usually is a kog maw spike but not for this case. Also because you already have the other speed runes, this won't be as necessary. also 10 extra ms after proccing it.

Futures Market. I am on the fence of whether approach velocity or futures market is better, however futures market lets you get spikes faster, and is very consitent. Whilest approach lets you brawl more.


Minor:

Attack speed because duh
movementspeed, as mobility is king, This plus transcendence you have 340 ms early without boots.
Scaling Health, because you get more value. IF you feel unsafe take flat Hp.

Ghost + Smite

nimbus cloak to run em down, even those that also rung host but not nimbus

with smite you have on demand "ghosting" and 5% ms

you can never miss a cannon again and cry because you are bad csing

You get tempo bases whenevery on cannon wave because of smite

you can surprise the enemies with a well time smite for Lvl 2 all in

after 20 minutes the cs penalty wears off and you can farm your junglers jungle while he clears waves( you are always ready to over other lanes, and are safer farming where the enemies cant see you)

IF you get this ******* item stacked

it takes ages

you get bonus shield from green, or nasty slows from red

counterjungling is enabled with smite and jungle item

IF you still want to go boots earlier than late, you may opt for triple tonic, or approach velocity

you could also go back to yellow for alacrity and triumph(or presence of mind)

OR you could go green for conditioning, and overgrowth to scale even harder.

Why not botrk, it gives insane damge, life steal and slows the enemies
its expensive af is why.
Also Kraken feels like more damage, , especially when they are close to being low
also the buildpath sucks

*wits end is ok because it’s cheap, and if you remember the ol times of wits rush, this was needed as it’s scaling now starts at lvl 9, with roa first and its level up passive later, you somewhat circumvent this as you get wits somewhat later.

*liandry and riftmaker may be interesting, but attack speed probably rules more/ hard maybe for 7th(when you sell roa)

*Swifty boots may be an option, but as the enemy team in 95% cases has at least 1 ad, Tabus are just so gooooood