What is the most efficient energy source available for human consumption?

What is the cleanest and most efficient energy source available for human consumption?
What is out there to light our cities that would produce little or no greenhouse gasses and still be viable? Obviously the sun is a clean source, but it's not efficient to capture its energy ... right? I mean, do we coat the surface of the earth with solar panels? Wind is clean, but again, do we have room for all the windmills it would take to make them meaningful? What is our best source of energy?