I'd probably go with UA (120K miles) or ANA with UA miles (130K) through NRT -- ANA has UA code-shares from JFK and IAD that would probably let you fly them and get away with the lower redemption level. Other reasonable options include: SQ with DL miles (140k), CX with AA Miles (135k), and JAL with AA miles (130k). Some of the European carriers provide more convenient connections (1 stop) from BOS, but their awards tend to be extortionately expensive.