Is green light most or least useful during photosynthesis? Why?

Photosynthesis is very high in the "blue" and "red" wavelength ranges and much less effective in the "green" region, and absent in all the other wavelength regions.

This is because chlorophyll, the principal pigment of plants reflects green light (500 to 600 nm) and absorbs maximally in the red and blue range of the spectrum (430 and 662 nm).

Plants can obtain all their energy requirements from the blue and red parts of the spectrum.