How do you define best internation calling card? It is not easy to define what is good. Is it the call quality? Is it price? Is it ease of use?
Then, depending on where you are, the cards are different. Cards available in one country/state may not be available in other country/state.
Then it depends again on your call destination. Some cards might offer very good quality for low rate for one destination, but bad quality and high rate for another destination.
So it is really hard to generalise that one is better that another. the best way to know is try more cards, then use the one that you like.
Adios 
p.s. If you are in Australia you can have a look at http://cheapcard.com.au/ . There is a good list of available cards there, and free tool to help you find the cheapest call rate.