What is the best dielectric?

No dielectric is the best dielectric.

That said, air would be my weapon of choice. 

Like @kijanki has said, an ovesized Teflon jacket is among the best. My  Tempo Electric speaker cables are one piece, solid core 14 gauge silver with a 9 gauge Teflon sleeve. Air becomes the primary dielectric and it's the best I've heard in my system.

All the best,
Nonoise

I would say what is the best jacket for WHAT PURPOSE?
Teflon is good for clarity, but not nearly as good for bass as some other types in insulation.

I refer to Kimber using a non Teflon insulation in products touted as have better bass response.
And in my experience, yes they do.
Kimber Hero, KS1116/1016 and Speaker cable VS.