Standard JLA was pretty neat. Speed, Strength Telepathy prep skills.
The "Galactic Storm" Avengers IMO were the most impressive line up power set wise: Hercules, Sersi, Thor, Quasar Cap, Ironman and Starfox are some of the best characters you can have in a team.
I would say the Young Avengers. They aren't the most powerful or experienced right now but once they hit their prime the diversity of their overall all power sets would make them a very balanced team. The only real hole in their line up is the lack of a high end telepath.
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FF, easily. None overpowered compared to the others, no overlap in powers, and everyone can be said to be useful. Pretty much no other team can truly say that. Their personalities also keep each other in check...a true balance through and through.
Because, realistically, 90% of teams (even good ones) are imbalanced because they have heralds chumming around with street guys that realistically have no business being there.
Kinda agree with BBZ. IW was Thanos, Warlock, & Friends. A good team? Absolutely. Cool team? Hell yeah. Balanced? Nah. Even ones with "specialty" powers like teleporting or telepathy could be recreated almost entirely by the big 2 mentioned. Hell 1-2 story arcs for IW specifically made the point of someone trashing the team only to have Warlock wtfpwn them.
My points still stand. IW was a giant showcase for how badass Warlock was, and how the rest of the team could occasionally help out.
Bats is actually one of the few exceptions to what I outlined above. Your Classic Avengers suggestion, however, is not an exception, nor are various JLA squads (though not necessarily all of them...I can rarely keep track of them all).
Well to me Standard JLA has been written to be the most balanced due to the nature of the characters involved. As in since they all involve popular well liked and money making characters everyone of them needs to be relevant and useful.
But if you look at a lot of their solo titles and factor a bunch of feats from outside of the JLA comics for various character. You basically seem to get a lot of crossover and redundancy in powers. And the forced balance writing of the JLA titles seem to go out the window.
Same with the Avengers. Thor, Cap, Ironman plus whoever else they have. A lot of the balance becomes forced when you actually look at the characters individual feats and titles.
I would say FF why because that team was put together with one goal in mind. To create a balanced team that needed each other to be able to accomplish all goals thrown their way. The X-Men also seem like a good choice for this.