Personally I am biased towards AS (Alaska), but then I am from ANC...
It is a good program to "dump" miles into as discussed before due to many
partner airlines, and if you do work in SEA or PDX it could work, as they have nonstops from BOS. They also have an excellent west coast network, and are slowly expanding their transcon service East and West... (BOS-PDX new, as is SEA-HNL (and LIH very soon)).
Redemption options are very good as well with several international partners on which to burn & earn.
Good luck and Happy Travels!