The asteroid belt, with robots. There's roughly a planet worth of raw materials, 24/7 solar energy. We just need to supply the labor force. We can collect, manufacture and deliver whatever we need to earth, or earth orbit in the case of spaceships, space stations and immense solar power arrays feeding clean power to those of us down here in the gravity well.
We need to perfect robotic exploration, mining, and manufacturing. We can use the moon as a first step. I say let's go back to the moon when there's a fully functional and pressurized moon base and fuel depot waiting for us on the surface. Heck, we can build and launch a craft to deliver most of the fuel for the trip from there and have it waiting for us in Earth orbit.