Web dating Køge
This is excellent, meaning your W active does 7% max HP at rank 5, but not as vital as the added attack speed from your Q.Finally, as your E is now so low in priority, take 1 point in it at level 4 and leave it there. Then W at level 18 to give you more consistent damage.
If you want Deathfire Touch (F:6.3), which is very good on AP Kog due to the range you can apply it at from your E (even though it's reduced effectiveness), then take Piercing Thoughts (F:5.2) before it.
However, I believe there's more value in both the Thunderlord's (C:6.2) options, so I personally go with that.
Throw one out there when you got a bit of mana to spare and you'd be surprised at the damage it does - 280 at rank 5.
The resistances reduction on it is a must against top lane tanks/jungle tanks/support tanks/anyone-that's-building-any-resistances, so make sure to hit the front line before you turn on your W in teamfights. The old Kog'Maw W gave a flat 2% HP damage at all ranks, but this one now starts at 3% and goes up 1% with each rank.
That could be their whole team taking 3% more damage. I understand why people go for Natural Talent (F:3.2) in Ferocity, 'cause it makes it easier to last hit, and gives you free AD and AP, but ultimately it's only 10 AD and 15 AP at level 18. For my money, the sustain from Vampirism (F:3.1) is MUCH better, at all stages of the game.