for me:
AA first cuz you can redeem almost anything and for me, most any time.
Plus there are so many good ways to earn many AA miles.
I use them a lot for BOS-to island destinations.
Next is NW.
For me, BOS-AMS non stop is key. This 50k flight can almost always be found for family and self when we visit relatives overseas. Cant beat that when you have little kids and time NOT being in connections or airports is paramount.
Then UA works for ski and west destinations. 25k still getcha there so long as you avoid CHI (unless going there for something)
I will fly the partners of said airlines but have chosen to earn miles on the carriers above.
MM