Ever since video games have been a thing, developers have been looking towards superhero and comic book properties for inspiration. The subsequent games that came from these licensed have run the range from being a complete disaster, to mindblowingly epic.
And, since the villains in the genre are generally super-powered, megalomaniacal and evil, it stands to reason that the boss battles would be tough. These examples are the toughest bosses from the best superhero games out there.
You may have found some of these more challenging than others, but in the end, we all breathed a sigh of relief when that evil bastard was put down.