The law of thermodynamics states that energy can't be created or destroyed. two natural sources of energy on earth are the
first, sun and core of earth
second, sun and rock
second, sun and biosphere
third, core of earth and hydrosphere

First, Sun and core of the Earth. The rocks have some heat because of the inner heat of the planet, and the energy created by other sources are far smaller than our molten core generates.

On the other side, if you're speaking about clean sources of energy we can take advantage from, I'd go for sun and biosphere, considering the solar and wind energy they most advanced and widespread sources we have.