With the $20m-estimated British Guiana 1c Magenta making its way to Sotheby's in June, the philatelic world looks set to crown a new king.  

Here are the top 10 most expensive single stamps ever auctioned, for now...

10. 1840 Great Britain 1d "Penny Black" - $345,131

The first ever adhesive postage stamp, the Penny Black had to make our list

Now, there are stamps that have sold for more than this Penny Black yet haven't made this list, but a top 10 just didn't feel right without the addition of the world's first adhesive postage stamp - the one that started it all.

The world record for a Penny Black is held at £216,000 ($345,131) by Spink, after the illustrious London auction house sold a mint condition example in 2011 from one of the first ever registration sheets, which was an imprimatur created before the famous stamp went into full production.

Other examples from the same sheet are housed in the Royal Philatelic Collection and in the National Postal Museum.

9. 1953 China "Military Post" blue - $428,654

A rare pair of the Military Post stamps, which were recalled for giving away confidential information

These magnificent looking stamps were first issued in 1953 in a set of three denominations, given to soldiers on active duty to send post home.

Bearing the logo of the People's Liberation Army, the stamps were soon found to be an easy way of identifying mail containing confidential military information and were subsequently withdrawn, with the remaining stocks destroyed.

Just 15-20 are thought to survive today, with a single stamp making $428,654 at auction in 2011.

8. 1897 China Red Revenue small "1 dollar" - $889,832

With only 32 examples of this temporary issue known, the Red Revenue stamp stands among the world's greatest rarities

The 1897 Red Revenue stamp, overprinted with "1 dollar" in small letters, is China's rarest stamp, and also its first ever issue, produced under the Qing government as it took control of postage from the foreigner-dominated Maritime Customs Department.

It was issued when China's currency switched from candarins to the dollar, but the writing was deemed illegible and replaced with a larger text. Just 32 examples are known to have survived the tests of time.

The record example of the stamp sold at auction for $889,832 at InterAsia in 2013.

7. 1868 United States Benjamin Franklin Z-Grill 1c blue - $930,000

Without the Z-Grill, this stamp is common, with thousands issued

Well known as one of the rarest stamps from the United States, the 1c Benjamin Franklin Z-Grill was actually the standard issue of the 1860s. However, very few are imprinted with the Z-shaped "grill" that gives it its name.

Most US stamps of the era are pressed with a grill to leave a pattern of indentations, allowing cancellation ink to take and to avoid forgeries.

However, the experimental Z-shaped grill features horizontal indentations that were found to be unsuitable for the issue. The grill was quickly replaced, leaving behind just two known examples. One of these sold at auction in 1998 for $930,000.

Despite being described as the rarest and most valuable stamp in US philately, the 15c Lincoln Z-Grill and 10c Washington Z-Grill from the same issue are equally rare.

6. 1918 United States 24c "Inverted Jenny" - $977,500

The 1918 24c Inverted Jenny is a fabled stamp, possibly even the best-known rarity in the world

The world's most famous error stamp, the Inverted Jenny, is so-called due to the upside-down image of a Curtiss JN-4 "Jenny" bi-plane - mistakenly printed on one pane of 100 stamps.

Dispersed throughout the philatelic world, these 100 stamps are some of the most sought after by collectors of the US, and regularly see top prices at auction.

In 2007, Robert A Siegel sold a mint single for $977,500, while a block of four made $2.7m in 2005.

5. 1847 Mauritius 1d Red Post Office - $1m

The stamp is based on the design of the Penny Black, though far more primitive

The 1847 Mauritius 1d Red was designed by Joseph Osmond Barnard, an Englishman who stowed away on a ship to Mauritius (not a bad idea) and went on to make a name for himself on the island, basing his designs on the famous Penny Black.

However, unlike those produced under the British government, Barnard's stamps are distinctly primitive, having been locally produced.

Just 500 were issued on September 21, 1847, with the majority used by the wife of the governor of Mauritius to invite friends to a ball on the tiny Indian Ocean spit.

The stamps were originally believed to have been an error, with the words "Post Office" thought to have been intended as "Postage Paid" by collectors. However, it was later revealed that the "Post Office" text was deliberate, and that "Postage Paid" replaced it in the following issue.

4. 1968 China 8 fen "The Whole Country is Red" - $1.1m

The Whole Country is Red stamp is one of the greatest treasures in People's Republic philately - ironic considering Mao hated stamp collecting

Since Chairman Mao decided stamp collecting was too much of a bourgeois pastime, the Chinese market for stamps didn't take off until recently.

It is now booming and, ironically, stamps issued under Mao Zedong are some of the most valuable in the country.

One such stamp is "The Whole Country is Red", issued in 1969 at the height of Mao's Cultural Revolution. It features a map of China in red, yet the island of Taiwan that was supposed to match has been left white - a mistake made during the design stage.

With China's government famously sensitive, the stamps were withdrawn almost as soon as they hit the shelves, The designer commented: "For a long time I was really worried that I would be jailed".

It's not certain how many of the stamps exist today, but they are some of the most sought after in all of China, with one example making $1.1m at China Guardian auction house in Beijing.

3. 1847 Mauritius 2d Blue - $1.6m

Almost the entire issue of 500 stamps was used by the governor of Mauritius' wife to invite friends to a ball

The partner of the 1d red, this stamp shares much the same story. The rarities were unknown to the philatelic world until 1864, when the wife of a Bordeaux merchant discovered a 1d and 2d stamp on an envelope belonging to her husband.

That envelope has since become one of the most treasured philatelic gems, known as the Bordeaux Cover. 

The cover was sold to the famous collector Philipp Von Ferrary, owner of one of the finest stamp collections the world has ever seen, and subsequently passed through the most famous collections, adding to their appeal for collectors.

An example sold for £900,000 ($1.6m) in June