Depends on what for, how much you spend etc.
For low expense, things like ING/Bankwest/Me/Ubank usually run % back cards, rotate between them so as not to pay fees and to take advantage of introductory offers.
If you spend a bit or go travelling overseas, more premium Amex cards usually get you more points and include travel insurance, concierges etc.
For when you are overseas you'll want a no fee card like 28 Degrees or Bankwest Zero.
For everything else you'll want to rotate between introductory offers with large miles/cash sign-ups where you can cash out before you incur fees that exceed the bonus.
