What is the best dielectric?

AFAIK foamed Teflon in oversized tubes gives the lowest dielectric constant close to 1 (air).

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

This is the most complete table of dielectric constants I have found.

http://www.clippercontrols.com/info/dielectric_constants.html

Anyone know a more complete source?

Oversizing should lower dielectric constant since air has lower DC than teflon. As for the difference with IC or speaker cable - capacitance of the wire is straight proportional to area and dielectric constant and reverse proportional to distance. Some teflon IC have as lo as 3pF per foot while typical shielded generic cable has about 25-30pF per foot. Dielectric absorbtion (proportional to dielectric constant and lowest for the teflon) is a process of storing some energy in capacitor's dielectric. Completely discharged capacitor can charge itself back recovering energy from dielectric. It is usually very slow and I don't know what effect it has on audio signals.

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