I'm just going to make this quick but from reading different guides and strategizing the best possible choices, this is what I have come up with. If you haven't played Kog'Maw before, he's not hard, just remember a few things. I'm not going to go into complete depth because most of this build is self explanatory.

1st. DO NOT spam your ultimate. If you use it again within 6 seconds of the first shot, it will cost more mana and it is very easy to blow all of your mana when using it.

2nd. Try to stay behind your team. Kog'Maw is a very squishy champ and this build doesn't have a lot of defense. Late game, the life steal will be essential.

3rd. Remember to Q and W. Using your Q while engaging will ensure that all of the damage from your W will do the maximum amount of damage.

The runes are pretty self-explanatory. The armor penetration runes are great for shredding through enemy champs. The attack speed seals are perfect for early dominance. The scaling magic resistance glyphs are a nice addition to protect from some long range casters. The quintessences are also nice for harassing and getting fed early game.

I have organized the mastery tree to be most beneficial to this build and used the mastery points as proficiently as possible. These masteries are great for doing a nice amount of damage while getting additional bonuses on the side.

After using a vast amount of Kog'Maw builds, this set of items will prove to be the best.

Doran's Blade-
The Doran's blade is a great bonus to help you out during the laning phase. Not only will it help you last hit minions, but it also allows you to do more damage to champs, get 100 health, and a little bit of life steal for sustainability to stay and farm.

Berserker's Greaves-
Getting the Berserker's early on is a great choice. Berserker's Greaves allows Kog'Maw to harass a lot using his Bio-Arcane Barage and his Void Ooze to slow.

Madred's Bloorazor-
Madred's Bloodrazor is an essential item for Kog'Maw.
It provides attack damage, attack speed, and some armor to top it all off. It also compliments you champ with a great unique passive. Combining Madred's with the Malady and the Wit's End is a perfect combination to lead you to victory.

Wit's End-
Provides a great attack speed and helps with sustainability with +30 magic resistance. Also provides a nice unique passive.

Malady-
A Malady is great for Kog'Maw because it provides some extra damage through ability power and it gives Kog'Maw a whopping +50% attack speed.

The Bloodthirster-
A bloodthirster is an exceptionally useful item for Kog'Maw. It provides great sustainability late game and I rarely see it included in other Kog;Maw builds. Without a Bloodthirster, Kog'Maw can be pretty useless late game. He will just get focused by the enemy champs once they figure out that you're squishy and you are doing most of the damage.

The Black Cleaver-
The Black Cleaver is one of Kog'Maw's favorite items. The Black Cleaver will let you shred, and I mean SHRED through enemies. By the time you get this, you're build will be amazing and the enemy team will be running away from you in fear.

Q-Caustic Spittle
Kog'Maw's Q is an alright ability. It's great for melting tanks and the attack speed passive can really help out mid to late game.

w-Bio-Arcane Barrage
Kog'Maw's W is a great ability that should be used very often. Bio-Arcane Barrage should be activated almost anytime you're hitting another champ. When paired with Madred's Bloodrazor, it has the potential to shred opponents.

E-Void Ooze
Definitely not one Kog'Maw's best abilities but it is perfect for sticky situations and when you are chasing someone with low HP. Don't necessarily use this ability to harass because the damage is very low and it uses a lot of mana. This ability is also perfect for landing a Living Artillery shot. You can use this to escaping by turning around and using it, or by using it in front of you and then running over it. Kog'Maw is not slowed by Void Ooze so don't be afraid to run over it.

R-Living Artillery
Kog'Maw's Ultimate is an amazing ability. You can use this ability to poke at enemies or to land the final blow to an injured champ. When harassing, try to put the shot behind them because when they hear the sound of the Living Artillery they will run back by instinct. This will actually cause them to run into the shot. When you are chasing, ensure that the shot is where they are running. As I mentioned before, make sure not to spam this ability. Wait 6 seconds before each shot, that way it doesn't cost more and more mana each time. Practicing with this ultimate over time is important. Start getting used to the timing of it and where to place each shot.

Exhaust and Flash are probably the two best summoner spells in the game. If you want to take Ghost to chase and escape, that's a very viable choice too. Exhaust and flash are extremely useful in many situations. Exhaust can be used while escaping, for engaging, and to chase. Not only does exhaust slow enemy champs, but it also reduces their damage output. Using Flash is great for escaping sticky situations and chasing enemy champs.

Teleport is an alright summoner spell for Kog'Maw. It can be very useful when trying to farm a lot. This can help get your items faster which can lead you to early to mid game dominance. When using teleport remember to farm as much as possible and last hit minions as much as you can

Ignite isn't such a great summoner spell for Kog'Maw, this is for two reasons. One, Kog'Maw shouldn't be in range to use ignite. While playing Kog'Maw you should try to exploit your range and keep your distance at a maximum. Two, using ignite on a dps champ isn't a great idea either.

Heal is also a bad choice because you should have at least one support champion on your team and a tank to take the damage. I suppose rocking a heal during the laning phase can be helpful if there is no support on your team and you are getting harassed a lot.

If you think Clarity is a good choice for Kog'Maw, you are not playing him correctly and you should read more of this guide. If you continuously run out of mana while playing as Kog'Maw you should make sure not to spam your ultimate, as each time you use it, it costs more mana. Also do not use Void Ooze to harass.

Smite & Revive, I don't even

PROS
-Tons of damage
-Very far range
-Ability to scout using his ultimate-Living Artillery
-Amazing harass

CONS
-Very squishy
-Can be a mana hungry champ

Farming isn't particularly important for Kog'Maw because you should be getting most of your gold income through killing enemy champs. But, early game during the laning phase you should make sure to last hit as many minions as possible and try to end up with a high creep score. If
you aren't good at farming or last hitting you can watch this video-

http://www.youtube.com/watch?v=6rZSI0Bdp6Y

Kog'Maw is a great asset to just about any team. Just remember to stay behind the tank. Try not to get focused by enemy champs and stay behind your team. Kog'Maw's range easily makes up for how squishy he is. Remember that your Living Artillery is a great tool to scout enemy champs and to check on dragon, buffs, and baron.

Overall, Kog'Maw is one of my favorite champs. I use him very often and I have the Lion Dance skin for him. Kog'Maw's damage output is insane late game. Combined with his range, Kog'Maw is an amazing champion and you should really give him a go if you haven't already.