Lolicat von Doom
To define the best superhero film one first has to define superhero. What is a superhero? Someone with extra-human abilities? Then Batman is out, unless we count keen detection as a superhero skill, and then surely we have to let Sherlock Holmes in? Does a costume make a superhero? Or is it a secret identity?
I think for simplicities sake, let's say it's a character who dresses up in an elaborate costume to conceal an identity. That seems to be fairly encompassing, and honestly, when I say "superhero," most people will think of Batman, Superman, and other similar figures.
(To actually define what a superhero is would take a careful analysis of the characteristics of modern superheroes, the evolution and introduction of the term in popular culture, along with underlying political critiques and many, many more variables. Needless to say, it's probably too lengthy a discussion to happen on this discussion board.)