table c: known specific heat values for common
materials
material specific heat (j/g*c)
water 4.18
concrete 0.88
wood 1.80
aluminum 0.90
glass 0.84
sand 0.83
steel 0.49
iron 0.44
copper 0.38
lead 0.16
gold 0.13
what is the best insulator and why?

Water

Explanation:

Specific heat capacity is defined as the amount of heat in joules needed to raise the temperature of 1 gram of a substance by 1 degree Celsius.

An insulator is a substance which conducts heat to a very poor extent.

Based on the two definitions, we may conclude that the greater the value of specific heat capacity, the harder it would be to heat up the material, that is, the more heat would be required.

Among all of the substances, notice that water has the greatest specific heat capacity, thus, it's the best insulator.

On the other hand, to make it clear, gold is the best conductor, as it has the lowest specific heat capacity.