The Sun. There are no clouds to contend with in space, you can collect quite a bit of electricity with solar cells out there. 24/7!
I suspect the winning design will have a very thin reflector focusing light on a solar cell, or maybe an array of cells.
Think along the lines of metallized mylar on a light framework. This is much lighter per sq. foot than the solar cells, and concentrates the sunlight. No need for a mirror with a perfect figure, we are only concentrating the available light.