Which material is the best resistor to heat transfer?

The best resistor to heat transfer in the context of this question is sawdust. Sawdust acts as an insulating material rather than a conductor of heat. This is because sawdust consists of small particles of wood that create air pockets, which significantly reduce thermal conductivity. The trapped air within these particles hinders the movement of heat, making sawdust effective at preventing heat transfer.

In contrast, metals such as steel, aluminum, and stainless steel are solid-state materials known for their relatively high thermal conductivity. These metals efficiently transfer heat due to the free movement of electrons within their structure. Consequently, they are not effective insulators and therefore do not resist heat transfer well.

Thus, sawdust stands out as the most effective material for resisting heat transfer due to its composition and the insulating properties of air pockets formed within it.