The sun, which gives us photons, units of electromagnetic energy as well as warmth, which produces wind, as well as the hydrological cycle.

The moon, the gravitational pull of which gives us the tides.

The radioactive decay of the earth's crust and the gravitational pull of each molecule on other molecules gives us the heat of geothermal power.

What is the best renewable energy source?
Based on the current technology and available options. Solar and wind are the best renewable sources.
The answer may appear controversial but it is based on current commercialized technology. Developments in material science is going to revolutionize the renewable energy industry.
We should also engage in a dialog about how to define and what to consider when we term an energy source as “best renewable source option”.
Recently “net carbon footprint” has come up in the discussions and my comments take that into account as far as the current technology and limitations.
You can find more information in these links, by presenting these links I am not implying that Solar and Wind are not “viable or best” sources, they are being presented to merely state where the current technology and challenges are.
I am very confident that we are very close to significant breakthroughs as far as efficiency and costs are concerned which will change the “new carbon footprint” of these renewable sources.