What is the best dielectric?

Kijanki - yep, I am fully aware of that and like I said they have their drawbacks.

You ask good questions, ones that I don’t know how to answer, except to say none at all. Being impractical, what I can offer though is the Fluorinated Ethylene Propylene (FEP) found in Nordost is better than most, but that is the gapped sleeve and monofilament (spiral wound/point contact) and air is actually the (better) majority of the dielectric. In my experience, these cables can’t be beat for tonearm to phonostage transmission. I haven’t found anything faster or less colored. Some may choose cabling as a tone control beyond this point, but source retrieval deserves the most transparent and honest of cables IMO.

Silversonic/DH Labs has a dielectric on their Air Matrix Cable that is a Teflon material that is actually 70% air.  Think of it as a less dense Teflon foam.

I believe Chris Sommovigo of Black Cat Cable/Stereovox/Stereolab and Kimber designs etc. used a braided foamed aeron material as well in his later Lectraline series, again lots of air for the dielectric present! 

Some reasons to admire a design such as Anti Cables are (1) no dielectric, (2) no jacket, (3) no shielding, (4) high purity copper and last but not least, (5) directional ARROWS.