Superman: the Movie is the template and high-water mark for the modern superhero movie, for its (mostly) serious treatment of story, its epic scale, its concentration on character and, primarily, for the performance of its star Christopher Reeve. Reeve said that he based his interpretation of Superman on a particular line of dialog which has him answer Lois Lane's "Who are you?" with the simple reply, "A Friend." Reeve's Superman was warm and kind and filled with heart, as if he were all of humanity's best friend, and his interpretation reshaped this American archetype forever.
The subtlety and power of Reeve's performance is never on better display than in a scene where, after feeling an extraordinary connection with Lois during their fly-over of Metropolis, Clark Kent arrives at her apartment and decides he will tell her the truth of his identity. He removes his glasses, his posture straightens, his chest expands, his chin sets, his voice drops several octaves and right before your eyes, a sniveling boob becomes a demigod. More than any other moment in any Superman (or superhero) movie, this encapsulates the power of the man and his secret burden and turns the demigod into an accessible human being.
Since Superman, the prime candidate for this legacy of humanity in superheroism is Chris Evans, who played the title role in the surprisingly great Captain America. After all, power balanced with humanity is the American ideal upon which the superhero was founded.
On that fitting note, have a happy 4th of July.