Highest Heat Capacity:

If you're asking about the substance that absorbs the most heat energy per unit mass for a given temperature change, then the answer is water.

* Water has a very high specific heat capacity, meaning it takes a lot of energy to raise its temperature. This is why water is so important for regulating temperatures in living things and in the environment.

Highest Thermal Conductivity:

If you're asking about the substance that conducts heat most efficiently, then the answer is diamond.

* Diamond has the highest thermal conductivity of any known material. It can transfer heat very quickly, making it useful for applications like heat sinks.

Other factors to consider:

* Phase: The same substance can have different heat-holding capabilities depending on its state (solid, liquid, gas). For example, water ice has a lower heat capacity than liquid water.

* Temperature: The specific heat capacity of a substance can vary slightly with temperature.

* Pressure: Pressure can also affect the heat-holding capabilities of a substance.

So, the best answer really depends on what you're looking for! If you can clarify what you mean by "holds heat best," I can provide a more precise answer.