Yeah Quiet seems to be the best character, by far. She's an interesting addition to the game, as much as I didn't like her concept at first, because of how she dresses but there's a convincing reason for her to do so.So in the end her character was well written.
I think that's kinda reminiscent of MGS 2 and how many expected to play the whole game as Snake, only to be forced to play most of the game as Raiden(and he wasn't even a badass ninja back then

). But here the idea is that Venom Snake becomes Big Boss, that we become BB in a way. But of course I think it could have been done better.
Because of this I think we need another game to cover the event from Big Boss' s perspective, or just right before the events in MG. In a way I guess that I understand how since it is supposed to be the last game in the series, it should be about BB himself, but we clearly didn't get that. So it leaves the impression that it was unfinished.