The most valuable coin in the world is the 1933 Double Eagle $20 gold coin of the United States - just one is legally in private hands at present, however actions are underway at the moment that are intended to see another 10 made available.
The first 1933 $20 Double Eagle sold at a Sotheby's auction in New York in July 2002 for US$7.59 million (due to the exchange rate at that time, the A$ equivalent was significantly higher) - I was lucky enough to get a copy of the catalogue from one of my former colleagues that attended the sale.
To quote another writer: "The history of the 1933 $20 in general and this specimen in particular is filled with ambiguity, controversy, deceit and, ultimately, triumph."
The issues the court case encompasses are broad and very interesting, particularly when there is potentially US$76 million at stake! I've got a couple of books on the history of the coin, the court case etc (admittedly they are in the "to be read" pile on my bedside table), however an interview with the lawyer taking it to the US Treasury Dept can be found here - it is fascinating to see this story unfold before us.