What is the best dielectric?

natural unbleached cotton is closest to air in dielectric constant.

That is a common misconception because people google dielectric constant and wind up here:

http://www.asiinstr.com/technical/Dielectric%20Constants.htm

a company that makes instruments for measuring bulk solids, the number given is for raw cotton very much like a cotton ball and has a high ration of air to solid.

This is a much more relative site for finding information on cotton as used in our field:

A Swiss textile company:

http://www.swicofil.com/products/001cotton.html#Properties

as you can see the dc for cotton made into a usable form, similar to what would be covering a wire is between: 3.9 – 7.5

what cotton does do well is reduce mechanical vibration which is why it is used frequently as a cable filler. It is also hydrophilic which is not something you want next to a metal conductor.

Microporous PTFE & PE and solid PTFE is for all practical purposes the best dielectrics to use. A tube filled with nitrogen would be great but not very practical to manufacture, a vacuum even less so.

microporous ptfe is 1.4 - 1.5 depending density, no oversizing, that's good for gases not solids, it will only increase dc..

AFAIK foamed Teflon in oversized tubes

Oversizing should lower dielectric constant since air has lower DC than teflon

The first quote would indicate oversized tubes filled with microporous PTFE, perhaps you meant, 'made of', which would go along with the second quote.

exposure to the atmosphere and having a loose conductor are draw backs to this but yes the dc is lower

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