Which material has highest thermal conductivity?

Thermal conductivity is the property of a material that refers to its ability to conduct heat. measured in watts per kelvin per meter (W•K−1•m−1). Thermal conductivity is reciprocal to thermal resistivity. Carbon Nanotubes have been found with highest thermal conductivity. However in some experiments thermal conductivity of Graphene was even more than Carbon nanotubes. In naturally occurring materials, Diamond has highest thermal conductivity.