Greatest Superhero is practically his definition. He is to superheroes what Conan is to barbarians, or Tolkien is to fantasy. He's the icon, the tradition from which all other traditions descend.
The people who dislike him tend to cite precisely his hyper-traditional archetypical attributes as a reason. He's basically too archetypically Superheroic for them. So whether you love him or hate him, all doctors agree, he's the One.
He's also basically Samson, Moses and Christ rolled up into one. He's the strongest, he's the natural leader, he's the quintessential character of inspiration, and he loves mankind, believes in us, inspires us, and will die for us. He's got hard-core saviour cred. He is the centre, morally and literarily, of the superhero universe.
Plus, he believes in 'Truth, Justice, and the American Way!' And despite not being American myself, I like that he's a superhero with a (mythologically) powerful and incorruptible integrity (not that people don't try, but the myth is greater than the stories), where he knows there is evil because he fights it every day, and there is good because he sees it every day, and doesn't buy that 'grey morality' or 'complexity' are actual answers beyond confessions of ignorance (not that that means the answers are always easy to find. But that's what makes it important. Truth is something you fight for).
He basically created and supports the entire genre. The question isn't 'Without Superman can there be a Justice League?" it's "Without Superman, can there be a Superhero Genre?"
If Superman was to die culturally, it would mean that we as a society no longer love superheroes. He's is iconic of everything that a superhero is. Even anti-heroes could not survive without him, because they would no longer have a tradition to subvert.
That alone, makes him the greatest.