For the record, I don't believe that Spider-man is the best superhero, but he does make my top five, so I'll still help you out. From the information you've provided, it's unclear what you mean by "the best superhero." Are you arguing that his character is the most intriguing or he is the most powerful superhero, or something else entirely?
He's certainly not the most powerful superhero, but there are a few good points as to why he's one of the more intriguing ones:
He's young, just a teenager when he first gets his powers. Before he was created by Stan Lee in the 1960's, nearly all superheroes were mature men.
Like many great heroes, his devotion to crime fighting is motivated by a great personal loss he has suffered because of crime (His beloved Uncle Ben being shot by a carjacker). But, unlike most superheroes, he unknowingly had the opportunity to prevent the situation, but chose not to. That lesson he learned the night his Uncle Ben died -- "With great power, comes great responsibility." -- is a key part of his identity.
Although he is not the fastest or the strongest superhero by any measure, Spider-man is one of the most acrobatic, which makes him very entertaining to watch, whether on the pages of a comic book or on the a TV or movie screen. He's also one of the wittiest superheroes, constantly bantering with his enemies as he fights them.
Compared to other superheroes, his story is relatively believable, especially since, despite his powers, he's still very human. In all the hundreds of Superman comics that have been printed, you rarely, if ever, hear any worries from Clark Kent except for his desire for Lois Lane, whereas Peter Parker is constantly worrying about his grades in school, photographing himself for the Daily Bugle, his friendships, the well-being of his Aunt May and his love for Mary Jane. I guess what I'm getting at is that Spider-man's alter-ego is much better developed that that of most other heroes. Also, he's also not bullet proof, he can't move mountains and he can't fly or anything like that, so he's also more human in that sense.
These are my favorite aspects of Spider-man, hopefully they'll serve as a good starting point for you.