Would you all agree that Sm4sh is the most balanced game in the series in terms of amount of viable characters?
Depends on if you mean by sheer numbers or by percentage of each game's overall roster.
If numbers, then absolutely - there's no question. The vast majority of this 55-character roster is well-balanced.
If percentage, then Smash 4 might fall just behind Smash 64 in terms of how much of its roster is well-balanced. To be fair, though, a 55-character roster is much harder to balance than a 12-character one!
No matter which metric you use, though, Smash 4 absolutely embarrasses Brawl and especially Melee in terms of balance. It's not even close.