Which Material has Highest Thermal Conductivity.

Hint: The capacity of a substance to conduct/transfer heat is referred to as thermal conductivity. It is usually represented by the letter ‘\[k\],' although it may also be represented by the letters ‘'$\lambda $ and ‘$\kappa $'. Thermal resistivity is the reciprocal of this parameter. Heat sinks utilise materials with high thermal conductivity, whereas thermal insulators use materials with low levels of $\lambda $.
Complete answer:
The rate at which heat is transmitted through a material is proportional to the negative of the temperature gradient and also proportional to the area through which the heat flows, according to Fourier's law of thermal conduction (also known as the law of heat conduction). This law may be written in differential form using the following equation:
\[{\mathbf{q}}{\text{ }} = {\text{ }} - {\mathbf{k}}.\nabla {\mathbf{T}}\]
The temperature gradient is denoted by $\nabla T$, the thermal flux or heat flow is denoted by q, and the thermal conductivity of the material is denoted by k.
Diamond (2000 – 2200 W/m•K) is the most thermally conductive substance, with conductivity levels 5 times those of copper, the most commonly produced metal in the US. Diamond atoms have a simple carbon backbone, which makes them an optimal molecular structure for heat transmission. The highest thermal conductivity values are often found in materials with the simplest chemical compositions and molecular structures. Many current hand-held electronic devices have diamond as a key component. Their function in electronics is to dissipate heat and safeguard delicate computer components. Diamonds' high heat conductivity is also beneficial for detecting the genuineness of jewellery stones. Small quantities of diamond can have a significant influence on the thermal conductivity characteristics of tools and technology.

Note: Lattice vibrations are largely responsible for non-metal thermal conductivities.
When temperatures are high, the mean free route of the phonons does not vary much, indicating that the thermal conductivity of non-metals does not change significantly.
When a non-heat metal's conductivity and heat capacity are reduced to temperatures below the Debye temperature, the non-heat metal's conductivity and heat capacity both drop.