Uranium and Radon, the densest solid and gas respectively, are the best conductors of heat. The ability to conduct heat is called specific heat and is closely linked to density. The denser a material is, the closer its specific heat is to zero and the more easily it conducts heat.
Other efficient conductors of heat include gold, tungsten, platinum and osmium. Elements that have a low density, and therefore are poor conductors of heat, include hydrogen, lithium, helium and nitrogen. Elements with very low and very high specific heats are used for their abilities to transmit or block heat effectively.