I've been to Sydney, Gold Coast and Brisbane. I'm also going to be a resident of the Central Coast, NSW in a few weeks, which is a very nice region as well, with a balance of rural/small and medium urban areas. Of those visited Sydney by far was my favourite, it had the most character and you could feel the history of the place, can't forget the views of the harbour and great beaches at Manly, Bondi etc......
I thought quite a few of Brisbane suburbs looked run down, particularly out East until you get to Wynnum, but its CBD is ace and their is plenty to do. Gold Coast looks class and feels like a vibrant, outgoing city but i don't think it is somewhere i would live. Looking forward to going the other capitals as well in time.
Most people say Melbourne is a better city to live in for infastructure etc.... but i think if the state government can sort out Sydneys roads and rail then it could be by far the better city