4. The Masters Of Evil
Now, I know this is a bit of a cop-out as the potential list of villains under the umbrella of The Masters of Evil isn't just one person, but like two dozen. But wouldn't it make sense for the 'evil that no single hero can face' that brings The Avengers back together be more than just one person? The Masters of Evil bring awesome potential as an adversary not only because of the 'war on multiple fronts' factor, but because having such a multi-faceted threat would bring the best out of each individual Avenger in terms of their own character arc. Instead of everyone just beating up on one guy, each protagonist could have their own individual tension with a focused antagonist, which I'm pretty sure is kind of key for creating effective and dynamic storytelling. Also, it means a bunch of heroes would be fighting a bunch of villains which, really, is just a good time. And that list of potential villains in and of itself reads like a who's-who of The Avengers' rogue gallery: Baron Zemo, Whirlwind, The Enchantress, The Executioner, The Abomination, Klaw, Living Lazer, Swordsman need I go on? Strength is in numbers, and the Masters of Evil would have been an awesome way to go before setting up the larger threat of Ultron in the finale of The Avengers trilogy because c'mon, you know it's going to be a trilogy.