So when it comes to lasers, violet lasers which have the shortest wavelength also have the most energy and are therefore the most powerful whereas red lasers with the longest wavelength also have the least amount of energy. With a rainbow, you see light in all its separate colors with red being the topmost color, with its longest wavelength it bends the least and so it makes up the top of the rainbow. The colors then go, in order of decreasing wavelength and increasing energy, orange, yellow, green, blue, indigo, and finally violet. Violet is the innermost color in the rainbow with the shortest wavelength and the most energy. Therefore a violet laser would be the most powerful.
So therefore, Mace Windu with his lightsaber that has a violet beam would have the most powerful lightsaber and Darth Vader, with his lightsaber which has a red beam would have the least powerful lightsaber.