So some people may say that my masteries have parts I can improve, but I will explain my page now so you can maybe understand my opinion.
You get all your power in this build just by AttackSpeed and it's the best choice since the most of your damage comes from your auto-attacks. Sorcery seems not really fitting here.
Players are arguing here a lot. Some say the 2% damage which seems really much isn't worth because youre taking 2% more damage.
I have to say that it actually is worth since is really squishy and you have to position yourself to survive like every ADC has to. You will die without positioning no matter you take it or not. This mastery point is spent well here since it's (in my opinion) superior to Feast.
This was a hard choice for me and I had to make a lot of tests and 2% Lifesteal and SpellVamp seems really nice,
but so does the extra AD and AP. Do as you prefer but keep in mind that actually has a good AD and a good AP scaling.
Again there is a bit more thinking with this one. Oppressor is a general good choice since you get a flat amount of % damage on enemies that are movement-impaired. Since you only have got your E to get this damage it's just not as good. You would have to rely on your teammates CC to get this damage.
Bounty Hunter gives you 2.5% more damage than Oppressor.
And this damage is constant without CC.
Since you still are a carry even in jungle it's also not as hard to kill every enemy at least one time, since your endgame damage is really strong and AoE.
Some ArmorPen which is better than MagicPen on you even if you do mixed damage.
Warlords Bloodlust is not worth taking since you need crits to heal yourself and this build doesn't contain crit chance.
Fervor of Battle is the best choice here because of your AoE damage in teamfights that stacks your bonus damage from 0 to 100 in the blink of an eye.
The movement speed out of combat isn't too bad for jungling since you can travel between lanes and camps faster, but the bonus damage on minions and jungle creeps you get is really much.
You may think its just 5 damage per attack, but if you count it, a lot of bonus damage comes up with it.
It's a hard choice. I really like Assasin and you can definitely take it for 1v1 situations. Since Kog'Maw isn't really strong in 1v1 against a lot of enemy junglers for example Vi and Elise, you should probably take Runic Affinity.
Secret Stash is also not too bad but I prefer the other ones especially because you won't get bisquits when taking a /.
It's nothing new. After getting killed you can walk for a short time. While this time you ignore every spell/cc etc. and nothing can stop you. You're not able to cast spells or autoattack.
The point is to walk to squishy enemies or a big group of enemies and let yourself explode there.
Damage is true damage so you WILL do the number the passive tells you. (100 + x 25)
Q - Caustic Spittle:
This spell is the thing your enemy tanks will hate the most. It reduces their Armor and MR to up to ~30%.
Imagine how much this is! Let's say has activated his W - and now has about 300 armor.
You will take ~100 armor away for 4 seconds. It's just so(oooooo) much! And not only your damage will improve with that. Your teammates do more damage to him aswell, since this is an debuff applied to your enemy and not an ArmorPen buff applied to you.
It's also your second choice in maxing your basic abilities.
At level 13 you should have it on level 5. Tanks will now have a huge problem facing you in teamfights.
W - Bio-Arcane Barrage:
MAX THIS! SERIOUSLY!
This is the ability your whole build relies on.
It passively increases your attack-speed with every level, so it's your first priority to get this spell on level 5 as soon as possible.
Get DOUBLE AttackSpeed and double your AttackSpeed cap from 2.5 to 5.0.
It also gives you AttackRange like it did before the rework.
Everything that isn't a minion gets 55% of your damage. Imagine a world in which you would do 100% of your damage with this speed. But you can definitely ignore this, you still deal a ton of damage and you're really OP.
And again this is an ability that will melt your your enemy team. And with melt I mean MELT.
You do bonus % damage, which is not only good against tanks.
Many people may forget, but you can chase people even better since your cast time on abilities are shorter. You won't stay at place that long while shooting your E,Q and even your R. You will be able to chase a lot better so even if youre out of range, do activate it for your R so you can get him while escaping.
E - Void Ooze
Nothing special. You will max this last, because the damage on AD isn't good and you only need the slow.
You could argue and say: yeah the slow becomes stronger when maxing it, but Q is still better maxing for fights and ganks. 20% slow on level 1 is enough to get closer to your enemy or to escape him. Trust me
R - Living Artillery
THIS. IS. SO. GOOD. NOW. !
Seriously. It's a fudging execute now and it's really strong if you want to get a kill far away.
Don't spam it early on in fights, because it will get expensive. You won't have much time doing this while spraying your sh!t around with your W.
Always place it where you predict your fleeing enemy to step into. He will predict you shooting Ults at him and he WILL dodge. So why don't shoot it where he would step into. (He could also predict this. Predict-ception! But I guess you get my point.)
Machete > Talisman on start. It's just better.
I also like the refillable potion since the corruption potion is really nice for taking care of your mana probs you may experience.
Warding totem for early on to feel save.
Probably the best choice on Kog'Maw Jungle. You don't want Skirmishers or an extra slow. You won't get in range for it.
Get sweeping lens when you get this item.
this is the shit. get it to sated state as soon as possible
it's the best choice. get boots of speed first and upgrade to this later after your first or second item
This is your core. providing the attackspeed, mix damage and aoe you need. since guinsoos is really broken this build destroys whole teams.
Get quicksilver if you have problems with cc. since mercurial scimitar has got a little buff it's almost worth taking all the time
These are actually really good last items as well. worth taking a look at these and trying out your favorite play style.
Take banshees when you have problems against champs like Morgana, thresh or blitzcrank.
general good last item. take it if you have to survive one time to turn the fight
It's worth taking especially in SoloQ since there won't be too much vision.
This item is really good because of the percent physical damage it provides. also the lifesteal and attackspeed makes this item worth taking. you can take it after hurricane instead of Mercurial scimitar
This item is actually really good on Kog'maw since it gives you a little bit tankiness and a really nice slow you can definitely use. worth taking as 6th item
-> -> -> -> -> ->
This is my favorite build and has the most team-destroying carry effect. Tanks can't stand it.
Build 2: (probably better)
-> -> -> -> -> ->
Really safe and provides a lot of utility and damage while having Mercurial Scimitar for good defensive stats.
Nice Build and probably the best to start with.
You should make sure to get your own playstyle since there is no chance of playing especially like somebody else.
Of course there are core items like Guinsoos and Hurricane in this case, but the rest is completely up to you.
Try my build and make your own decisions and your own opinion.
-Kog'Maw's basic attack speed has been nerfed from 0.665 to 0.625.
The lost of 0.04 attackspeed may seem small but it's still mentionable, since it's the best stat Kog'Maw can stack. Don't worry! Kog'Maw is still strong as an ADC and as a jungler. You probably won't even notice the nerf.