Depends on how you define "well-balanced roster".

I would say Big 4 is easily the most important in the sense that you can usually round out your team with welfares and then for most quests, you can borrow your friend's OP DPS of choice, pair them with Mashu + one of Merlin/Waver/Tamamo/Skadi and viola, you will clear the quest. Since you have your own supports, you can run easy farming clears and save time on that as well. The only place you will have issues is Shimousa with its boss fights which don't let you borrow friend supports. Easier to focus on with limited SQ if you are F2P and bonus points if they happen to be your favorites(good thing that all 4 of them are favorably liked outside of gameplay).

K-scope, ehh. It is of course amazing but in front of your choices, it pales in comparison since you will get enough 50% battery event CEs which are usually more than enough with the right support.

Well-balanced squad is fine too if you mean proper DPS options in each class. That is the other way around of Big 4, wherein you are almost always borrowing a friend Waver/Merlin/Tamamo/Skadi but you have your own DPS of choice so more customization. If you are F2P though, chasing this will get really costly.(Unless you are relying on welfares for this, in which case why not big 4?)