Kog'Maw as an ADC
I noticed that there weren't many build guides for ADC Kog'Maw, so I decided to share the one I use.
At level 18 with this build you'll end up with about 4.5 attack speed using w, which isn't that bad at all. I WOULD build Kog'Maw full 5.0 attack speed, however I find that building him full crit with some damage ends up going over way better then going straight A.S.
I'll quickly go over the items I picked for this build and why I picked them.
Cull: With your "W" ability it's really easy to farm minions, especially after you get Runaan's Hurricane. Cull exploits this and ends up giving you a decent boost pretty early on.
Blade of the Ruined King: Building this item first gives you a fair amount of early attack speed (which is the core stat in Kog'Maw's kit) and some decent damage to make use of that attack speed.
Runaan's Hurricane: This and BORK are your two core items. Runaan's gives good attack speed and some crit chance which scales really well on Kog'Maw because of his W. Also, the two extra bolts help in team fights when you're sitting at the back lines machine gunning the three opposers (bolts help A LOT with farming, too).
Berserker Greaves: Kog'Maw is one of the least mobile ADC's there is, so to prevent Mr. Rengo from sneaking up and 1-hitting him, you take boots for some running away speed. Berserker's Greaves are the obvious choice for boots as they grant attack speed.
Statikk Shiv: This item gives you a fair amount of attack speed, crit, and some movement speed which can't hurt. The passive is amazing on Kog'Maw because with his 4.5 attack speed using this build (that's 4.5 attacks PER SECOND) the statikk shiv fill up really fast.
Rapid Firecannon: Like the statikk shiv it gives you attack speed crit and movement speed. The passives on Rapid Firecannon and Statikk Shiv stack, so his 4.5 attacks per second really do a lot with these two items.
Essence Reaver: This is a good finishing items as it rounds off your build with a bonus 65 attack damage, 20% crit to get you up to 100 (10% is wasted, however I don't think there's any better item to go here) and a total of 30% CDR with Essence Reaver's awesome passive (this makes his w have a 4.6 second cooldown about).