I'm pretty sure that the best player on the best team is not an explicit criterion, though some do think along those lines. There have been many years when the best team was in fact the best team because it had the best player. That logic makes sense. In 2 of the years cited by Nan and Bajan, the leading scorer wasn't the best player--Angel and EDD--but they were probably the second best players. In other years, the leading scorer wasn't even a top-10 player.
I think that Kelsey Plum is the clear leader. Why? Her scoring is obviously a consideration, but not a guarantee. Scoring loads of points and also shooting .500, .400, and .900 is rare and noteworthy. She is carrying her team to a high national ranking. Plus, she is approaching the all-time (under NCAA) scoring record. That's a very impressive resume.
I was the one, in anther thread, who was speculating as to what would have to happen for Plum not to win. I think it would take a number of things. First, Kelsey would have to miss several games (God forbid!) or have a substantial slump. Another possibility would be for Washington to lose several games and drop in the rankings. Beyond that, someone else would need to make a big splash in some way. I am the one who said that, if Lou scores 30 pp. from here out, she will get consideration. I still expect Kelsey to win, but there are possible scenarios under which someone else does. Lou is playing her way to be very high in the NPOY voting, possibly #2.