What is the most important lesson you learned as a writer?
When determining who the POV character is in any given scene, go for the character who is under the most stress, physically or emotionally. I use this rule every day and it works 90% percent of the time.
(A notable exception: murder mysteries, where often you can't go into the heads of certain characters without giving things away. Makes POV issues more complicated.)
To give credit where it's due, I learned this from the late Terry Carr, a veteran sf editor and author, and privately think of it as "Terry's Rule."