The best color is no color. Diamonds allow light to be reflected and dispersed as a rainbow of color. The light dispersion has no effect on the technical grading of color.
The absolute finest colorless stone carries a D rating, descending through the alphabet to Z, designating a diamond of light yellow or brown. These gradations are so minute and precise that discerning a single grade (even by an expert) under less than ideal laboratory conditions is virtually impossible.
It is often surprising to learn that diamonds also occur by rare accidents of nature in shades of pink, blue, green, amber or even red. These rare occurring colors are referred to as fancies and are evaluated by a different set of color standards.
Color scale courtesy of the GIA.