A movie where the superhero is the most interesting and dynamic character
A movie where the "essence," the core truth, of the character is well-presented
A movie that is reasonably faithful in translating the character's world and supporting cast to the screen, while also acknowledging that some things must inevitably be altered because film and comics are different media
IMO the best superhero movies I can think of are Iron Man 1 and Batman Begins. They check all the criteria I listed above.