Often times the best miles for a specific region pair is the one using which you can find availability.

But for "optimal" use, another to add to your list is:
N America <-> Australia: Delta. Delta gets access to Virgin Australia award space, and it's generally the best availability, particularly in premium cabins, for NA-OZ. DL charges a little more for this route than UA and AA, but you can actually find seats, often multiples on the same flight.