It's by chronological order of when they fell in love (or the closest thing to love he and Sekai had), so while it is true that he first met Noriko when he started to see Niki, she was just a child and we know that Akira isn't that bad of a person. They fell in love only when he started giving lessons to her as wellIf this is by order they met, Noriko should be before Maya. If it's by importance to Sensei, she should be further down based on how the last events have gone for her. Maybe just below Noodles the bird.
Also, I don't think at all that Noriko isn't important to Sensei. In fact, the last events show us that he doesn't fuck her exactly because he cares deeply about her, compared to other fucktoys like, let's say, Makoto or Futaba