What is the best dielectric?

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.  

No dielectric is the best dielectric.

That said, air would be my weapon of choice.