Depends on what your expectations are for 'best'.
You have 3 main carriers and a bunch of virtual operators. The best carrier is Telstra but it can be the most expensive. Their network is the best for speed and coverage however if you're only going to stay in Melbourne, all 3 carriers will be fine. If you go bush, go Telstra.
I personally use Optus with their $2 days. $2 gives you unlimited calls, texts and 500mb of data. If I need more data, an extra $2 will give me 500 more megs but you're capped after that for the day. For days where I just use wifi, I pay nothing.
Not many carriers give you large amounts of data. You'll find 10-15 gig options from Yatango and Amaysim who are virtual operators (they resell Optus I believe). I believe Amaysim is 3G only but Yatango should be 4G as well by now (if not now, soon). Those will cost you around $70-$100 off the top of my head.
There are other options, including data-only plans but most come on a plan that lasts 12-36 months. I probably have missed something as the last time I did research was late last year.