Interesting question smoothdancingirl... although I don't really think there is a "the best."
Aside from hair color I'd also say that skin tone would have to play a major factor in any such determination. Beyond such predefined variables, however, are the external, venue related factors.
Bright or dark ballroom? White lights or colored lights as well? If so what do blues and reds (lighting wise4) do to the color of the gown? What color is the floor? Light or dark provides for different contrasts as well as different amounts of reflection from the ambient light sources.
Of course some of these variables tend to stay constant at any given venue so, for instance, I know several people who plan the shade of their costuming based on floor conditions...i.e. have multiple dresses but know that one of them is just too dark for "x" event, etc.
Then, of course, there's also the question of what other competitors have on as well. You might have on a fantastic dress but, if 4 other women in your heat have the same color on, you might be better served in a less "ideal" (for you) color, but that will set you apart from the rest.
Just some thoughts...