Kog drops off late game
Since Riot removed the total AD ratio on Kog's W and gave it a flat value (break even point was 160 total AD. So a stacked Rageblade + runes) Kog lost his hyper-carry curve. Previously he just kept getting better and better into late game. Now he peaks at the 35-40 minute range and drops off hard the later the game runs.
I was a bit hesitant to make a topic about this for a while since there's little data on Kog due to how few play him, but this has been a consistent pattern in every single patch since the total AD ratio was removed. And so here we are.
As I see it, Riot can do one of two things here. If they want Kog to be a hyper-carry, they need to revert the change they made to Kog's W. Bring back the total AD scaling. Keep the 15 minimum damage that's on the PBE, though, that'll prevent him from being totally worthless at level 1.
However, if Riot doesn't want Kog to be a hyper-carry, then keep the 2% base damage and make the minimum damage scale strongly with points in W to give Kog a much stronger mid game.
Either way, Riot needs to figure out what they want Kog to do. Because right now he's got a shitty early game, a decent (though not amazing) mid/late, and a meh everything else. At no point does the risk in picking Kog really pay off.
as core. Especially nashor's seems imprtant fot me to get the cdr(also useing cdr/lvl glyphs) for a better W uptime, and usually sorc boots.
The other two items are situational, but some of the more common items for me:

as core,
as situational pickups
Especially ER is quite strong,30%cdr help his W uptime alot, and it prettymuch gives him infinit mana for the ult.