I like a 5 hero team with Legacy, Chrono Ranger, Omnitron X, Argent Adept, and Haka.
Legacy and Haka can tank while dealing admirable amounts of Melee damage.
Argent Adept, Omnitron X, and Chrono Ranger provide a variety of Ongoing destruction.
Legacy and Argent Adept can buff everyone else in various ways.
Omnitron X and Chrono Ranger can dish out a surprising amount of damage over the course of the game, especially if Chrono Ranger gets his "Ultimate Target" bounty in play early on. Combined with buffs from Argent Adept, Legacy, and Ultimate Target, Chrono Ranger's Composite Bow is dealing 10 damage every turn the villain deals damage (which can destroy villains with a redirect like Blade and Iron Legacy, who will likely be dealing damage on each hero turn). Omnitron X's components can be fragile, but he has a good number of ways to get his Platings out on the field, so as long as you know the damage type you're up against, you can minimize the risk of his components being destroyed.
Argent Adept and Omnitron X can allow other players to play cards out of turn, helping them to get set up quickly. AA's ability to let players draw cards and use powers out of turn can really aid in dishing out some quick damage as well (or to let Legacy double Galvanize, making Chrono Ranger even scarier).
And what does Haka do? Well, everything else. Because he's awesome. Ground Pound or a well timed Rampage can completely turn the tides and Savage Mana can allow for some trippy battlefield control. He could probably be replaced with another hero, but Haka gets it done.
Is the team perfect? No, there are probably other combinations that work far better. If you want a perfect team that will give you the easiest win, having Wraith and Visionary for near perfect villain control is probably ideal. But, I don't have nearly as much fun with them for that very reason.