This article should help you understand what is going on (the video gives the basics):
The big reason why your NBN is slow, and why it might soon change - CNET
As to which provider, it varies from area to area depending on which POIs various RSPs spend their CVC dollars. For example some RSPs focus on metro areas and make sure their CVC quota is adequate in those locations - where most of their customers are. If your RSP does not have many customers in your area, they may not invest that much in CVC at the POI (point of interconnect) that services your area.
Aussie Broadband (NOT Australia Broadband) has had a very good reputation. That being said Telstra is also reputed to be one of the better ones too - perhaps not for your area.

