The POV should probably go to the person with the most "stakes" in the scene, the person who experiences a bigger "life impact" or "state change" to their character arc. That is the character who will most likely be the focus of reader sympathy.
This is not necessarily the most vulnerable or most emotional, and it may not even be the one who is doing the most action (or having the most action done to them), rather this is the character who – whether by participating or observing – experience a turn in their story arc development (not necessarily the one with the most plot or action).
For example: In a final contest between rival Characters A and B. Character A is the handsomer wittier athlete, charismatic and the favorite to win. The POV might go to Character B because she has a realization about the purpose of the contest, and it changes how she views the system that rewards winners and punishes losers. Or it might go to Character B when she realizes she doesn't care about winning, or that she would rather loose fairly than cheat to win. The "plot" seems to point at A being the more important character, but B is the one who is having a turning point in their character arc.
In many cases, the POV goes to a less important character because the more visible character can't tip their hand to the reader, think Watson and Holmes. Holmes is cleverer and smarter, but Watson experiences more "stakes" because he does not know the outcome of the mystery.
Coming of age stories probably follow the least experienced and least important person in a household, but who has the farthest to grow, or who makes a life-altering realization.