Justice League might have been first, but some would argue the Avengers did it better. At least, Earth's Mightiest Heroes have always had some diversity in the characters. The roster isn't stacked with a bunch of characters with the same abilities and everyone brings something different to the team.
But first, here's the history of the Avengers, per Movie Pilot.
Inspired by DC's well selling Justice League of America title, the Avengers first graced the pages of Marvel comics in September of 1963, in The Avengers. Like the Justice League, the member roster has changed over the years, and the Avengers have had a huge turnover in members comparatively, and many spin-off titles such as the New Avengers and the Secret Avengers.
The original Avengers were Ant-Man, Hulk, Iron Man, Thor, and the Wasp, and they were brought together by Hulk's sidekick Rick Jones to combat the machinations of Loki (it's always Loki, isn't it?).
The Avengers brought together the best of Marvel. The variety of character types made them quite the powerful team, with many lineup changes over the years—simply because Marvel has so many quality superheroes to choose from.
The team boasts a super-solider who's body is at peak human condition and strength; a giant green rage monster who's arguably one of the strongest beings in the cosmos; an immortal Norse God who can summon lightning; a genius billionaire playboy with an army of high-tech armored suits; a Russian spy gifted in martial arts; a mad scientist who can shrink and grow to any size; and an archer with a quiver full of trick arrows.
Can't you just see the variety of skills and powers only a few Avengers add to the team? That diversity of talent would easily defeat the Justice League—a team full of copycat heroes.
Check out below just why the call them "Earth's Mightiest Heroes."