Argent Adept, Team Leader Tachyon, America's Greatest Legacy and Nightmist
Argent Adept, Team Leader Tachyon and America's Greatest Legacy have amazing synergy together. Both AA and Legacy can give Tachyon extra power plays, which in turn gives the whole team loads of extra cards (this really helps both AA's and Nightmist's setup time). AA and Legacy also have a lot of synergy, when Legacy gives AA an extra power, that power can cascade depending on how well AA is set up.
AA is a fantastic supporter and makes the team better, his powers also mean more than most other powers, simply because one power can actually end up triggering multiple Performs and Accompanies that add up to much more than any other power.
Nightmist is very versatile, extremely difficult to take down and really appreciates any support thrown her way. She has good damage output, deck control and survivability. Nightmist + Legacy give a good chance of being able to keep the Villain from doing much for one or more turn. She doesn't need the extra power as much as the other Heroes do, but she can use the extra draw and card plays in a huge way.
Team Leader Tachyon has great damage, some extremely useful control cards, and while some complain that she can't get as many burst cards into her discard pile as base Tachyon, I've never run into that problem. While the original often has issues getting her important cards out early thanks to an unlucky draw, or she plays so many cards that she runs out of them very quickly. TLT on the other hand halves the time it takes to get those necessary cards and all but guarentees that she won't run out of cards to play. All the while helping out the entire team. On this team in particular, she will get burst cards much faster than the base version. And she gives the team anywherefrom 4 to 12 [or more] cards a turn.
America's Greatest Legacy has almost all of the awesome damage, survivability, support and versatility of base Legacy while adding powerful synergies with TLT and AA. Any hero team without some form of Legacy is really missing out. His only weakness is deck control (though he has some powerful options there too, like Take Down.) As much as losing Galvanize hurts, on this team I prefer America's Greatest Legacy.
They have pretty good options with Ongoing and Environment destruction, great deck contol and will always have the cards they need (or be a turn or two away from them, at the most).