Hmmm, BA does not have "chart" so to speak - because effective Nov 16, BA award is both Distance-based and Segment-based - with that it means, the award is based on how far you travel, AND also how many segments you fly in order to get to your destination.

Simply because it is distance-based, no one in his right mind would even consider using BA Avios for a long distance trip such as BOS-SYD - not to mention that you would have to connect at least once, may be two or three times (each would add one segment thus add cost) to get there - instead of being the "best" as you proclaim, it is probably the Worst among all other options.

What you really should do as a starting point, is to read each program's award chart so you know how many miles are needed for a one-way business class award to Australia. AA is one option - you can redeem it on QF flight as a partner award, so check the All Airlines chart on AA website. With AMEX points, your option is limited because their transfer partners that would be of interest for this trip would be Aeroplan, BA or DL - both AC and BA would charge huge fuel surcharge for your trip, and BA costs an arm and a leg even on the miles portion as explained above. Good luck in finding anything on DL... UR points would be a more feasible option when you can transfer it to CO, which allows some liberal routing to redeem award. Again, you would check CO's award chart, and if you have a CO account with some miles, just try for a dummy award search and see for yourself.