4) Masters Of Evil
Whereas the Justice League have the Injustice League, the Avengers have the Masters of Evil. Founded by Baron Heinrich Zemo (the father of Helmut Zemo, the founder of the Thunderbolts), the Masters are a union of several of the Avengers’ recurring enemies.
Typically, the Masters of Evil are comprised of secondary level villains who become a whole lot more dangerous when they’re part of a team. These include Black Knight, Radioactive Man, Enchantress, Scorpion, Absorbing Man, Yellow Jacket and many, many more. Some versions, however, have been led by other major MU antagonists such as Doctor Octopus, Ultron and Doctor Doom (at least in the Marvel: Ultimate Alliance video game).
The version of the team led Baron Helmut Zemo is the most successful, however. Under his guidance, the Masters of Evil managed to destroy the Avengers Mansion in the classic ‘Under Siege’ storyline. They might often be defeated by Earth’s Mightiest Heroes, but that doesn’t mean the Masters of Evil don’t cause a whole lot of damage in the process.