I'm an Aussie, so Telstra. ;)
We have a few choices in Australia, the big players are: Telstra, Optus and Vodafone. All the other smaller ones like Virgin Mobile are resellers based off one of those three.
I used to be with Vodafone but decided to move after a big outage occurred in Western Australia where both voice and data services were down. They were selling really cheap post-paid and prepaid plans and as smartphones got more and more popular in Australia it got to the point where they just couldn't keep up with capacity, making it super slow for everyone.
As Aurora2698 mentioned, Telstra is more pricey but it just works. :)