Greetings, all. This is my first build, so please comment and vote. No troll voting, if you don't mind. Now, let's get down to business... it's time to feast!

Kog'Maw is always fulla pros.

Pros:
Crazy fun champion.
2.5 attack speed is amazing.
In my opinion, best passive in the game.
Also, one of the best ults in the game. In my opinion, anyways.
This build should tear apart even the toughest opponents.
Valuable in teamfights and ganks.
Superb anti-tank.

Cons:
Oh so very squishy. If you get focused, you are likely to die.
Not as good as a balanced Kog.

The purpose of this build is to utilize Kog'Maw's inherent attack speed, HP shredding from Bio-Arcane, and armor/MR lowering from Caustic Spittle in order to quickly render opponents vulnerable for you and your team. It will max out attack speed and apply so many on-hit effects, it's not even funny... it's hilarious.

Summoner spells I take with Kog'Maw tend to be:
for getting around the battlefield. Generally, I swap this and Smite depending on the champion.
is for mana. What? You say it's useless? Well, frankly, no. I understand how it is with other champions...BUT Kog'Maw's when spammed properly and used in conjunction with his other spells tends to deplete mana at a rapid rate, and can dry out even the largest reserves. It's nice to be able to click a button and essentially load another round of Living Artilleries up. Also, those mages who were confident that they had enough mana and mana regen, but didn't... they will thank you. Also, it grants some major superiority early game.

But if you REALLY disagree...
can be used to further focus down an opponent, especially if you decide to move the crit point to Cripple.
What can I say? Who doesn't love Flash?

Items are built for on-hit effects to weaken the opponent, as well as get attack speed to the 2.5 cap.
lowers armor,
lowers MR,
is essential on Kog'Maw, and shreds HP, and
slows with each hit.
Put all of them together, and the man you mark should go down easy.

Alternatively, other options include:
if you feel you could sacrifice one attack speed item for another. The 100 dmg every 4 hits coupled with Malady has potential. Also, it helps against dodgy opponents.
is one I considered. If lifesteal is your bread and butter, then go for it.
if you just love stacking on-hit effects. I suppose the Grievous wound can be helpful in some situations.

The purpose of this build is to shred through tough opponents. As such, focus on a particularly strong opponent, and get your team's damage dealers to lay him out. Alternatively, if their squishy opponents just don't seem squishy enough, the only sensible solution is to make 'em squishier. In any case, the team should focus the person you target.
IF YOU ARE BEING FOCUSED: Don't be an idiot on this one. See that Bio-Arcane Barrage? It lets you stay well out of range of most champs, and STILL lets you apply those wondrous on-hit effects. FURTHERMORE, if you really need to stay out of range, use Living Artillery. Hit 'em with the end of Void Ooze. When you run out of mana for your artillery, Clarity Reload and fire another volley.

This is most viable in teamfights. Probably. It may need improvement, so feel free to comment. No trollvoting, as said before. Constructive criticism is highly valued.
Zerglinator out.