I am currently with 3 and think they are great. I use to work for Optus, thus had my phones with them. Not long after i left my job Optus went down hill big time. Back then 3 were known to be bad. However.. now, alongside Vodaphone (as they are now technically the same thing) i believe 3 are one of the best. We live in a remote location. I have perfect service out here, whereas DH with Optus (and eggarly awaiting his contract to expire) never has any service. The thing with 3 and vodaphone now is, if you dont have coverage with their particular towers, as a default you go onto Telstra's towers. So the coverage is just as good, if not better, than telstra.
So in my opinion, either 3 or Vodaphone atm are the way to go.