What is the best dielectric?

Ah, but a tube filled with liquid nitrogen would be even more intriguing, especially if the wire it surrounded was made from one of the high-temp superconductors. Imagine the look on your buddies faces when you topped up the liquid nitrogen in your cables before your local audio club's next listening session! Mmwaaahaahaahaa!

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.