What is the best dielectric?

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

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.

Cpk - there are several cables made with oversized tubes (most of Audioquest, Acoustic Zen). I'm not sure if tubes have air or nitrogen inside. Material of the tube itself can be polyethylene, foamed polyethylene, teflon, foamed teflon - depending on the price of the cable.

Muralman - additional problem with speaker cables is skin depth. There is no skin effect down to gauge 18 within 20kHz audible range but my cables are gauge 7 (necessary or not). Splitting conductor into many (isolated) helps since surface area is increased but effect is still there (combined magnetic field is increased). Now the trick is to arrange wires in pattern minimizing addition of magnetic fields. Audioquest does "Hellical", others have different sometimes strange looking schemes. Acoustic Zen has over sized Teflon tubes (air tubes) with multiple conductors (about 10) plus one with multiple strands. Weave pattern is sort of hellical.

Have you tried Au24 - it has very thin dielectric?