In my opinion, solar energy is the most sustainable renewable energy source. All forms of energy are derived from solar energy, so the most efficient and low impact source will be energy sourced directly from the sun. All the earth's geologic cycles get their energy from the sun. Fossil fuels are simply concentrated chunks of solar energy.
Solar energy is also one of the most water efficient of all sources of energy, which is very important in a globalized world facing population increases and water shortages. The only water needed for solar energy production is to clean panels or mirrors a few times a year.