You pay your money and you make your choice. Federer and Nadal had the benefit of homogenisation, Laver the benefit of having his play sharpened by the pro circuit and maybe catching the amateurs cold. Agassi the benefit of SEVEN years to do the job, and not the strongest grass fields ever.
What you can say is that each of them did it, while the players around them didn't. So none is unique, but all have this special achievement.