To find out the best use, you would have to look at then number of BA or AA miles required for the legs you are taking, and comparing that to what you would pay for a ticket on those legs.
As I have noted, you can also use miles for OneWorld awards. If done with AA miles, this requires flying 2 carriers other than AA. OneWorld awards are mileage based, not based on the number of legs... although I believe there is a max number of segments The OW website has an interactive map that can advise on members that fly specific legs.