Superman is the greatest because of his undying love of life. His ability to see the good in humanity when no one else can. His compassion and respect for people motivates him to do the most good, while everyone else seems to have some trauma trigger them to become a superhero out of vengeance. Yes, Jonathan Kent died, but he wasn't brutally murdered causing Clark to wear all black hockey pants and throw plastic bats at bad guys. No, even if Jonathan was alive, Superman would still be Superman. He saves the world for the sake of saving the world.