The first statement about the sky being blue is true more often because it is a scientific observation. The sky is blue because particles in the atmosphere most commonly reflect blue wavelengths of light that make the sky seem blue to human beings' optic nerves. The second statement about Obama being the best president is pure conjecture based upon public opinion and not scientific principles.
The sky is blue every single day. When the sun is out, the sky is blue. In sharp contrast, Obama has presided over the largest and most invasive domestic spying program in the history of modern civilization. He has also assumed the authority to kill American citizens without trial through drone strikes.