Pyrolitic graphite along cleavage planes is one of the best solid materials. Of course the best is superfluid helium.

What heat transfer mechanism?  Conduction, convection, radiation?  A vacuum is pretty good (or bad, if your prefer) for the first 2 but not the last....

Vacuum.

I am discussing about solid material not vacuum

You may want to refer to the "Thermal Insulation Handbook" (2001) from the Thermal Insulation Association of Southern Africa (TIASA), or to some other publication of this kind, such as: https://www.elsevier.com/books/thermal-insulation-handbook-for-the-oil-gas-and-petrochemical-industries/bahadori/978-0-12-800010-6