Who makes the best copper cables?

I looked again at the NRG material menu...if you want insight on everything about copper and cable build/construction for audio...this is a great 2 page overview...

https://www.nrgcustomcables.ca/my-materials

I have used several cables over the years and the cables I've enjoyed most are Audio-Envy and now Iconoclast.  Audio-Envy cables are inexpensive, and I believe $ for $ would give you great enjoyment and return on your investment.  Iconoclast do better than Audio-Envy cables but not by much and while more expensive, are really high quality in their construction and sonically as well.  If you look at how the Iconoclast cables are made, there is sound science behind them and testing with state-of-the-art equipment to prove it as well.  In any event, I hope you find cables that satisfy what you're looking for in sound.  

I have not seriously auditioned cabling in a while.  I consistently like speaker wire and interconnects from Audio Note.  The sound is predictable with their cables—on the warmer side, with their silver cables having greater clarity while still being warm and free from high frequency harshness.  The copper cables are a bit warmer, which I like, but are a bit murky sounding compared to the silver wire.  As you go up the price range their cables get better but the price is scary.

bthrb4u

May I ask about the Gear and Speakers in your system?

Happy Listening!

Another +vote for HIJIRI Million Series cables/cords.

Happy Listening!