What is the best dielectric?


A vacuum is the best dielectric? Since this is not pratical would air be the next best dielectric or would there be a better sounding dielectric like cotton?
Would there be a difference for speaker wire or interconnects?
Thanks
cdc
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.