Heat is the most random form of energy. This is because heat is effectively the kinetic energy of atoms or molecules vibrating randomly in three dimensions.
One of the fundamental principles of thermodynamics is that all systems tend to the state of lowest and most homogeneous energy distribution, which corresponds to the state of highest disorder. The random thermal vibration of molecules or atoms at a constant temperature represents this minimal state of energy. All forms of energy tend towards random dissipation in the form of heat.
Additionally, while all other forms of energy can be converted into heat with 100 percent efficiency, heat cannot be converted into other forms of energy with this same efficiency because of its inherent disorderliness. While it is easy to randomize existing energy into heat, additional energy must be expended to convert this heat into other, more orderly forms of energy, such as electricity. The loss of energy in the form of heat also manifests when converting between two energy forms, neither of which is heat. Some loss of energy in the form of heat most also be incurred. The comparative difficult of ordering a shuffled deck as opposed to shuffling an ordered deck is an illustrative analogy.