A few factors to consider:

- Your country of residence (FF rules, elite eligibility, awards, often vary based on this)
- Your preferred use of awards/benefits (free travel for self, upgrades, etc.)
- Your travel patterns (class of travel: F/J/Y, routes: SYD-BOM or intnl, Aus domestic, US domestic, Intra Europe, etc.)

If your goal is simply free travel in coach and you do not expect elite status, the choice of FF program will depend on the routes and class of travel for awards. From your query it appears that you are flying coach probably on something less than full fare. If so, different airlines give you different miles based on fare class. With the exception of certain QF and IB fares, AA is usually the most generous in granting miles on coach fares. For Biz/F all airlines are pretty standard (LH, NOT in OneWorld but in star is an exception to this practice. They give 200% miles for all paid biz travel). However AA does not fly much in Asia or Aus and BA has a better network out of India, CX in Asia and BA/QF for Europe/Australia.

If you live in Canada or have a Canadian address CX is generous and AA has many promotions. This is because they lost a network partner (Canadian, now part of AirCanada and Star Alliance).

If you do a lot of Biz/First travel then BA, AA, QF, CX are all great. BA probably helps you reach Gold fairly quickly (you can do this with one RTW biz) and AA EXP offers some unique benefits within the OW (though other US airlines have similar elite benefits). EXPs are given 8 VIPOWs that let you upgrade on AA network but not on BA, QF, CX or others. AA gives 100% mileage bonus for Platinum while on BA you need to be Gold not Silver. Also, note that usually airlines allow you to upgrade only on their network using their miles, while on other partners you will not be able to upgrade. (Exceptions are AA upgrades on CX, etc.)

There is now a OW award in Coach, Business or First that allows you to fly multiple OW partners with certain restrictions. Most OW partners have awards on each other. (In other words as an AA member you can get awards on almost all OW flts with the exception of BA transAtlantic). This means that you can use miles in any program to travel on most any other OW partner.

In short, I don't think there is any single "Best" but a few different optimal choices based on your patterns and preferences.