Hai :D I'm A Tad Facetious, and this is my Kog'maw build. I decided to make this build because I like to play Kog'maw and the current builds out there make him to squishy for my liking. (either that or I'm just noob)

WARNING: This build works well for ME and MY play style. Using this build does not Garen-tee (<--see what i did there ;D ) you will win.

1st guide ever btw :3

Pro's of Kog'maw.

High damage
Consistent damage
solo ability
Revenge passive

Con's

Until level six you will be fighting at your turret usually (that's if you are soloing against two)
squish mitten until you get life steal.

I went with these runes because they are standard Attack Damage and Attack speed runes. Although allot of Kog'Maw's Damage comes from ability power, the base for his W skill (Bio-Arcane Barrage) comes from attack damage. The armor and magic resistance will let you lane longer and not have to worry about taking damage from your lane opposition, this will also mean you dont have to submit to harassment. which is good :D

With my itemz i like to build speed first hen i build into damage, i like Madred's because along with your Q you will be 10% of their max hp's in magic damage roughly 2 times a second, but you still have the damage from your melee itemz when your q fails, this build also means that along with magic damage you will be doing alot of melee damage. with this combo your enemy will need to waist alot of money buying resistances items giving you an advantage.

These Masteries are for leveling fast/spell damage which helps allot with Kog's Ult and lane presence, but they will also serve you when it comes to survivability, your time spent in lane, where about in lane you play (i.e. at there turret or yours) and your mobility.

I like to take Flash and Ghost in my build because it allows you to engage in the all important team fights allot faster, but lets you escape from ganks when the time comes.

ALTERNATIVES: instead of ghost i could recommend taking teleport if you have friends learning to play a character or if you feel you need the extra maneuverability.
You could also take ignite instead of flash to make sure they die but i find flash more helpful for me tower diving given the long range that your W gives you as well as your ult's range.

PRESENCE IS EVERYTHING! don't shy from combat if there is equal teams, indecision will kill you and your team mates.
DON'T ALLOW HARASSMENT! With your "W" you can out range most abilities (except for skill shots, which you can block with minions) if someone comes to hit you, hit them back. your team members should be close enough to save you from melee as well.
FRONT LINER: Kog'maw, although others might say different is a front line character, with your life steal and raw damage you have amazing survivability, and if you die it allows you to set your passive up a lot better.

Most importantly, and this factor will decide whether or not you loose your fights. SMILE AND WAVE!!! if you are mean to your team they will play a lot worse, if a team is playing bad on purpose you CAN'T WIN!

Lots of love The Crimson Derp (Thats me in games <--)