The diagonal is easiest to create, but hexagonal is best packing. Since 6 circles fit around a central circle of the same diameter, you can fit 6 doughballs around a central one. The only issue is whether the height to width ratio of the cookie sheet makes hexagonal optimal, or results in the same number of cookies as diagonal. (Someone will surely produce a table of width/height ratio by arrangement? Assuming the smallest diameter dimension is at least N cookie diameters?)