What is it about Batman that makes him one of our favorite superheroes?
To understand the essence of the Batman mythos, we need to go back to the beginning and understand Batman's origins. As a child, he wasn't that special, just another spoiled rich boy. And the single event of his parents' murder changed all that. Not only did he now have a reason to take vengeance on the criminals -- true vengeance, not goody-goodness -- but he also had a reason to be truly afraid of the world.
I have always liked the fact that Bruce Wayne chose the bat as his symbol: the thing that he feared the most. If you think about it, the symbolism is beautiful. Bruce is becoming what he's afraid of: an angry vigilante. Only his self control keeps him from becoming what he hates more than anything: a criminal.
As I see it Batman is half-superhero, half-villain.
Batman is special precisely because of the conflict between his ethics and his anger. Superman wants to save the weak little human race. Spider-Man is just a nice guy with some guilt issues who happens to spin webs. Thor is some divine being thrown to Earth and does some nice stuff for us. Captain America is just too good to be true.
Batman is one of the most human and flawed of all the superheroes out there.
This fundamental conflict between one's own goodness and evil is so rich, complex and relatable.
Batman is the man that we wish we could be and are afraid to be. He has lost his faith in law enforcement and the judicial system. Many of us have lost that faith as well. Batman struggles with the idea that he could go bad at any moment; we do as well.
And so Batman is our superhero.