I have just come back from a holiday in the Clare Valley, so I might be a bit biased, but I visited Taylors http://www.taylorswines.com.au , Pikes http://www.pikeswines.com.au and http://www.jimbarry.com/ I would recommend these to anyone. The Mornington Penninsula, was beautiful also - great Pinot Noir.

That question can't be answered any more than one can state which are the "best" French or Italian or Spanish regions. There are many regions in Australia, some of which are yet undiscovered. Keep in mind that Australia has the oldest soils in the world, they have a unique situation since there is nothing between them and Antarctica except the ocean, and they have not explored half of the areas that may be good for wine producing. Margaret River is a relatively newer area, somewhat cooler, and it's vastly different from say, Barossa. Coonawara is another that can produce stunning wines. So you really need to define your preferences first, and then you can gauge better which regions may suit you or strike you as best. Cheers.