The most stable phase of a substance at any given temperature is the phase with the lowest

free energy

Under different conditions of temperature and pressure, the physical states of a substance changes and these variations can be graphically illustrated through phase diagram. As we cross the lines or curves on the phase diagram, a phase change occurs.  At low temperature, enthalpy dominates and solids has the lowest enthalpy. When temperature is high, entropy dominates and gases has the highest entropy.

Free energy is characterized by both enthalpy and entropy. The balance between enthalpy and entropy is controlled by the temperature and spontaneous changes always lead to a lower overall free energy.