Peter Parker, the science geek that gains the power of a spider. I loved that story as a kid. The thought that a person could climb up vertical surfaces, hang from ceilings and swing through the streets of New York was intoxicating. Even now that I am a middle aged that feeling hasn't gone away.
I've never really read the comics, just watched the films, cartoons and TV series so how Peter gains his powers varies, is he bitten by a radioactive spider or a genetically engineered spider? In some versions he produces his own web in others he makes his own chemical that fires out of metal web slingers. Whatever the story, the powers are fab.
However, the main reason that I like Spiderman is the line, "With great power comes great responsibility.". This statement reveals what Spiderman stands for - something good.
If I could be a superhero I would be Spiderman.