... from an historical perspective, at The Prudent Investor:

Time-travelling from the old Greeks to the Roman empire, to the British empire and then the young history of the USA one notes that the most widely accepted (reserve) currency always had its homebase in the political powerhouse of its times. The political power always footed on three determining factors: The productive capacities of that nation, its international trade relations and its capability to defend itself.

While there were several denominations of silver coins in circulation in the Greek empire that had their origin in the provinces of Byzantine, Macedonia and Peloponnesia, to name just a few, the Roman empire first introduced the silver drachmae in order to facilitate trade with the Greeks. The drachmae was followed by the golden aureus, the silver denarius and the bronze sestertius. One aureus was equivalent to 25 denarii or 250 sestertii.

The aureus had a respectable lifespan of more than 400 years before inflation diminished its reputation. Nothing has changed since: Whenever a currency loses its value, so does its popularity...

The reserve currency of the 18th and 19th century was undoubtedly the British pound sterling...

With the demise of the British empire which went hand in hand with the outsourcing of its productive capacities to the colonies where labour was cheap, the pound was replaced by the US dollar in the early 20th century when the US ascended to the throne of the biggest producing economy in the world, a place it has held since.

Looking down the road, TPI (aka Toni Straka) wonders if the number one contender isn't China, rather than Europe:

Most forecasts see China becoming the biggest economy on the globe by 2020, give or take some setbacks on the way to there that are inherent with such growth rates that this country has been enjoying recently.

China still buys time on the way to a liberalization of its currency controls, not least for the reason that its financial sector is still in its infancy, China will also develop this sector and gain knowledge on that way. With a consumer base of probably more than 1.5 billion people by then it has a huge backyard on which it can rely for further growth for its growing international importance. As the country has been on the path to a more liberalised economy for the last 15 years, taking one step at a time, its careful planning of the future will lead to a more prominent role in the capital sector. Those who produce can save too and therefore become a supplier of capital needed elsewhere.

Regardless of whether China's emergence is likely in the short-run, Toni is on board with the idea that the dollar may be have peaked:

Staying with dollar based prices could become a too costly way for others to conduct their bilateral trade with the currency of a third country that is primarily known for its huge amount of debts and no plausible recipe for a turnaround of this situation. After outsourcing its production an outsourcing of control of international trade could be well on its heels. The race for financial dominance is on.