The first thing to note about Kog'Maw is that he does not fit into every team composition, and in ranked play is easily countered by high damage champions with a reliable gap closer. Kog'Maw has no escapes outside of Flash, and is very easily 1v1'd by champions such as Akali, Xin Zhao, Nocturne, etc. If you are picking Kog in ranked play, make sure you do so after noting that the enemy team has no strong method of interfering with your heavy damage output, because they WILL want to focus you. That being said, Kog'Maw is a very strong AD carry and can potentially have the highest DPS of any ranged right click carry. His biggest strength, however, is the ability to deal massive damage to High health targets, such as tanks, or the dreaded "Atmog's" bruiser. Ensure that you are selecting Kog'maw alongside a team that consists of plenty of CC, and that can easily protect you.

The runes in this build are fairly self explanitory. I do not believe that Kog needs mana regen, so I stuff Armor into his seals, and MR into his blues, giving him much needed defensive stats for his the laning phase. I go with Magic Pen reds, because the damage from ALL of his abilities do magic damage, and he also deals magic damage through Bloodrazor and Wit's End.

I leave the Quints up for grabs, as Kog'Maw does well with most of the standard Quints that you would run on any other champion.

Mastery selection here is fairly cookie cutter for any right click champion, outside of picking up the Magic Pen, and neglecting the crit damage mastery, as this build does not feature a large amount of crit. I almost always go 9 in utility for the sweet experience buff, and the extra gold per 10 is always nice.

Bio-Arcane Barrage is your primary damage ability. It gives his Auto attack an incredible damage boost, and should be maxed first. I pick up Void Ooze at level 1, however, for it's teamfighting potential if there were to be an invasion in your jungle, or the opposing team's jungle. I pick up Caustic spittle at level four, then prioritize R>W>E>Q. This will give you the most Damage/Lane control while you are farming.

ITEMS ARE ALWAYS, ALWAYS, ALWAYS SUBJECT TO CHANGE. That being said:

The build presented is in a "perfect situation" for me. If the enemy team is heavy on CC, Mercury Tread's is a MUST, PERIOD. Outside of that the damage items listed are self explanitory. Kog'Maw THRIVES off of his rightclick, and buffing that with Bloodrazor and Wit's End is the best way to boost his sustained damage. I typically go Black Cleaver next, as the armor shred stacks very quickly with his massive amount of Attack Speed, and this will not only benefit your auto attacks, but also any other physical damage champion on your team. After that I like to get Frozen Mallet, which has a very nice bit of health, a bit of damage, and a slow that gives your Auto attacks some strong utility when chasing or kiting. Last item is always an iffy for me. If I'm doing well and not dying, I'll pick up the Gunblade for lifesteal, spellvamp, and the AP that buffs your q, r, and ult. Note that the AD from your Bloodrazor, Cleaver, Mallet, and Gunblade also scales with your ultimate.

Kog isn't the best champion for landing kills in lane unless you're laning with a heavey CC partner. That being said, your primary focus should be farming. His auto attack animation is very good for last hitting, and his W can harass the enemy carry off of last hits if used properly. Don't overextend yourself, as Kog'Maw is very vulnerable to ganks from the enemy jungler. In the current Meta, you'll typically be duo-laning bot with a support. Make sure they're warding for you, and feeding you last hits on minions, as it is essential for your team's success! Once you hit 6, you can start putting out some very heavy harass, and can start trying to get kills if your lane partner is competent. Just remember not to overextend, and play it safe. Farming is always your number 1 priority. NEVER sacrifice a last hit for a little bit of harass, that gold is way too important on such an item dependant carry.

This is where team composition comes into play. Positioning is the single most important part of your success in a teamfight. Constantly poke with your ult as the teamfight is initiated, and once you see that the enemy team has committed and that you are safe, feel free to press W and shred the enemy carries with your massive range. Don't forget to drop your Q on them, and laugh as you deal ridiculous amounts of damage. Save your E for when you know they won't be attacking you and you're safe to use it offensively, rather than defensively. His ult is great for securing kills from low HP enemy champions that are running from the teamfight, and it is also great for zoning enemies from towers while your team is pushing.

Pros:
-MASSIVE DAMAGEEEEEEE
-Highest damage to tanks of any AD carry
-Heavy zoning potential with his Ultimate
-MASSIVE range, and can teamfight very safely if played correctly

Cons:
-Very vulnerable to ganks while flash is down (no escape)
-A large amount of his damage is magic, offering your team less damage variety, and making it easier for the enemy team to itemize properly
-Very squishy
-VERY expensive item build