Which of these materials is best for a ground rod driven into the earth?

An effective ground rod must possess two essential qualities: high electrical conductivity to safely dissipate currents into the earth, and high physical strength to be driven deep into the ground without bending. Copper-clad steel is the ideal material because the steel core provides the necessary mechanical strength for driving into hard soil, while the thick outer layer of copper offers excellent electrical conductivity and resistance to corrosion. Pure iron or steel would rust quickly and have lower conductivity. Hard plastic and fiberglass are insulators, meaning they cannot conduct electricity to the ground at all, making them completely useless for electrical grounding.