Cheapest isn't always best (so avoid Dodo). Forums such as whirlpool are worth looking at to see if there are many complaints of congestion in specific areas. Typically the cheapest ISPs have fairly limited backbone for the number of users they have. Complaints of practically unusable internet is certain areas at certain times is a common problem. Better providers generally focus on having enough backhaul and infrastructure for the number of clients they have. Cheaper ones won't, and peoples experience with them varies greatly.
Ultimately it depends on how important reliable, fast internet access is to you, and at what price. Cheap =/= the best, and the best ISPs typically aren't the cheapest. I'm with Internode who arguably run the best network in Oz, but they're not the cheapest. I have a nice fast path to San Jose which important to me as a I have gear there, so I get nice, low latency. They have quite a few international peers. There are others there that use them too. From a technical view point, their network is pretty impressive. The link below will shed some light on what people tend to think of their service.
I'd suggest checking out
http://whirlpool.net.au/survey/2010/ to compare other peoples mileage with specific providers. If you do some research on the ISPs who offer things like unlimited downloads, check where those ISPs fit in terms of service and reliability - typically they have higher rates of poor performance and reliability.
(for extra points research the number of providers who have done unlimited long term in Oz and see what's happened to them - the cost of moving data in this country makes it quite clear that this is not sustainable)
Just to complicate things, everyone's milage varies. I could put a heap of people on a crap connection who think it's great, and could put one person on a good connection who thinks it's average - will depend entirely on how you use it, when you use it, where you are, and what you're comparing it against. That's why some people love providers like Dodo and TPG - both of who are considered to have the worst services but the cheapest prices. Some of their clients love them, some use them for a week and need to move elsewhere (and some wind up leaving with things go pair shaped).
So - it's really up to you to decide between the price you're prepared to pay, what you want, and how reliable you need it to be. You won't get as many options with cable, but it's probably a better option than ADSL if you can get it (at least in terms of the speed you can get out of it).