So I started playing Kog'maw recently and I've been using a build I came up with that seems to be fairly effective, but I'm unsure if it's the most optimal way to build him.
My first buy is 
, followed up by a 
. After that I get 
and then complete 
.
After that I usually go 
> 
> 
.
My main concern is the lack of Last Whisper, but I'm unsure on which item I would replace. Any suggestions?
EDIT: Also, would hybrid Magic Pen + Armor Pen quints and marks be a good choice?
You really want to choose just ap or ad. The best build I find is ad, rushing trinity force first. If you find you need mana build the sheen first and continue, otherwise start with zeal. Then you will want BotRK, Berserkers Greaves, and probably Last Whisper. Finish with IE, Phantom Dancer, or a def item like GA or Banshees. Bloodthirster is another option but I find not the best on Kog.
sooner (before ,) see if it ups damage output midgame.Feels like you're delaying your damage a LOT with this build.
is always cost ineffective early game (but worth for late game, if you get it maxed)
is kinda 'meta' but I think it's at least mildly situational. (you want someone with a decent chunk of HP stacking before it's worth..otherwise, 
might be better for the shield..and siege strats..and, kog is good at sieging)
Putting last seems odd. The damage boost for that is insane, waiting for the +50 to crit till max items makes me feel like you could be carrying harder earlier. Should at least try bumping the priority of it up a few slots for a couple games and see if it's better.
Also for straight single target damage, 
is better than now. Consider getting it if you're not trying to get the AoE damage out in team fights and just focusing down someone.
If you're looking for a straight replacement for 
, maybe drop or possibly even and and get something else instead. No reason to force yourself into a crit based build, Kog isn't really a crit required champ. 
are options. Though that's going into a whole other build.
could replace though, extra mana and cdr from it goes well with the , just gotta weave more spells in-between autos for it to be worth. But then you might feel like you're getting into Blue Ez build.
ok my adc runes and masteries are always 21/0/9, get ad runes with 1 quint then 2 lifesteal, then armor and mr. Build Kog like Corki or Ez get TF first then Blade, if they stack armor get last whisper or start building ap. You shouldn't first pick kog unless you have mobility support (karma, lulu, thresh) so you can kite and poke and not get caught out if you team can support sieging and poking then he's a good pick and you should only pick Kog if they have a tanky comp and you need a hyper carry because Ez and Corki are better due to escapes and less mana intensive ults.
for ad kog i have a weird build kinda but here's what i do
i like to go for first, then get a now after these two people complain about my build alot because i like to get a then a or an then one of the following: 
and i buy boots somewhere in or before the first two items
