Introduction:

Kog'Maw is my favorite champion for numerous reasons. Appearance, background story, abilities, not to mention he can be an insane beast. This build is what I like to call 'FaceMelter', mostly because of the look on my friends face when Kog'Maw melts them in 4 seconds flat. For the most part, this requires the user to carefully choose their actions. Never forget you are a squishy!

Runes:

Greater Mark of Alacrity: Bio-Arcane Barrage is greatly boosted by this, and it is needed to get to 2.5 attacks per second.
Greater Seal of Replenishment: This one is a personal preference, as I like having my mana fully recover between eating enemies. Never know when you need to spam Living Artillery.
Greater Glyph of Focus: Another one that I think personally is a no brainer. Less downtime between activating Bio-Arcane Barrage is something to definitely love.

Masteries:

Must Take:

Archaic Knowledge: If you don't take this, you are INSANE.
Utility Master: Needed so you keep those neutral buffs longer.
Presence of the Master: As your summoner spells are for survivability, this is an obvious choice.

Avoid:

Greedy: 1g/10sec = 6g/1min = 360g/1hour... waste of a talent.

Skill Sequence:

I prefer to pick up Bio-Arcane Barrage and use it to harass enemy champions every chance I can early on. Especially since it puts fear into those that stack hp or tank. Stacking Caustic Spittle is also useful for the passive attack speed bonus, which makes your Bio-Arcane Barrage hit harder.
At least take one point into Void Ooze so you can better control your enemies. You don't need to max it early on, you will be lethal long before then.
Take Living Artillery every chance you can. It will be crucial in taking out those pesky fleeing champions, especially since you don't need to see where it will hit, just time the shot well enough!

Summoner Spells:

I have had people ask why not Clarity, why not heal, and so on. Honestly heal will not save you. Clarity will allow you a few more shots, but is situationally useful. As is almost every other summoner spell in the game! These two provide you with the biggest boost to survivability available.

Items:

I always start out with vampiric scepter, to give me some way of offsetting the damage I know I will take from minions at least. If you play defensively enough, this minor amount of life steal should allow you to stay in the lane a lot longer than your enemies.
On first trip back to town, you should have enough to buy your boots of speed and emblem of valor. Maybe even your berserker greaves.
On second trip, recursive bow and Madred's Razors. Nice boost to attack speed, and you should get more last blows on minions due to the razors 15% passive 500 dmg.
On third trip, you will be picking up your Madred's Bloodrazors.
On fourth trip, grab a Void Staff. With your Archaic Knowledge, you have roughly 49% spell pen, meaning even your enemies who stack MR, they are still doomed.
On fifth trip, Feel free to grab either Guinsoo's Rageblade, or Stark's Fervor. Both would give you attack speed boost to push you to 2.5 attacks per sec, Stark's is more for survivability, the Rageblade is more for damage.
On sixth trip, Rabadon's Deathcap. If nothing else, this just makes you more lethal.

Summary:

1. Building up to 2.5 attacks per second. This means after 4 seconds, you've done 10 attacks. Remember this, it is important.
2. 25% Life Steal gives you survivability for when someone tries to sneak-attack you. Also prevents people from burning you down easily.
3. Bio-Arcane Barrage does 6% of target Max HP as magic damage, plus 1% per 100 AP. End build is 413 AP, meaning 10% of target Max HP per shot, plus 4% from Bloodrazor.
4. With each shot taking off roughly 14% of your target's hp, within 2 second, you have done close to 75% of their HP. Even if they heal, 4 seconds is all it takes to melt enemies.
5. Haven't tested this, but if you have the attack speed boost to spare, you can swap your Berserker Greaves for Sorcerer's Boots, or Boots of Swiftness, etc.

If any changes are made, I will be posting credits for the changes.