Silver has the highest thermal conductivity of any metal under ordinary reference conditions. At room temperature, pure silver conducts heat at roughly 429 W/(m·K), allowing thermal energy to move through it exceptionally efficiently.
Its high conductivity comes from silver's metallic structure and mobile conduction electrons. The same electronic behavior also gives silver the highest electrical conductivity of any metal. Copper is nearly as effective at conducting heat and is far less expensive, so it is used more widely in wiring, heat exchangers, and many engineering applications.
Silver's thermal performance does not mean it is the best conductor among all materials. Diamond conducts heat better than silver, and unusual states of matter such as superfluid helium can exceed it as well. The ranking also depends on purity, temperature, crystal structure, and how a material is measured. Gold is an excellent conductor but conducts heat less effectively than silver, while aluminum is lighter and cheaper but also lower in conductivity.