Penny Black
Issued in May of 1840, the Penny Black stamp was the world's first prepaid adhesive postage stamp. The stamp bears the profile of Queen Victoria against a black background, framed by a finely-engraved border that bears the word "Postage" above Queen Victoria's head and "One Penny" below it. In 2011, a used Penny Black stamp was worth $240, and an unused stamp could be traded in by collectors for more than $3,000.
Hawaiian Missionaries
First issued in October of 1851, the Hawaiian Missionaries stamps were the first used in the Kingdom of Hawaii. It's a simply-designed stamp with a bordered numeral in the center, "Hawaiian Postage" at the top, and the stamp price written out at the bottom. All were printed in blue ink on pelure paper, a thin and fragile tissue-like paper that meant few of them had a long life. An unused stamp was worth $735,525 in 2011. A used version was worth $367,700.
Basel Dove
The Basel Dove was first issued by the Swiss canton of Basel in July of 1845. Designed by the architect Melchior Berry, this stamp features a white embossed dove that is carrying a letter within its beak. The stamp was designed and printed in black, crimson, and blue, making it the world's first three colored stamp. In 2011, this stamp was worth more than $19,700.
Post Office Mauritius
Also known as the Blue Penny/Red Penny stamps, the Post Office Mauritius stamps bear the profile of Queen Victoria. They were issued as in two colors to differentiate two prices: "One Penny Red Brown" and "Two Pence Blue." Released in September of 1847, these stamps were used in Great Britain and Ireland, and were the first British stamps to be produced outside of the United Kingdom. In 2011, one of these surviving stamps was worth around 1 million pounds, or more than $1.6 million.
Inverted Jenny
The Jenny Stamp was a 24-cent stamp created specifically for air mail delivery in 1918. Because air mail delivery was a costly, the stamp cost quite a bit more compared to the average 3 cent stamp. Each stamp features a blue Jenny plane framed by a red border. The Jenny Stamps were printed in sheets of 100 that had to be run through the printing machine twice to print the two colors. During this process, one of the sheets was accidentally printed with the Jenny plane upside-down. An inverted Jenny sold for $525,000 at auction in 2005.
Red Mercury Stamp
This Austrian stamp from 1856 was used as postage on newspapers. It features the profile of the Roman god Mercury surrounded by a simple border. Although the stamps were printed in blue, yellow, and red, the red stamps are the most valuable because they were only printed between the years 1856 and 1858. Because only a few copies of the stamp exist, each stamp was worth $40,000 in 2011.
Treskilling Yellow Stamp
The Treskilling Yellow Stamp was the most expensive stamp in the year 1996, perhaps because there is only one surviving specimen. The Treskilling Stamp was released in 1855 in Switzerland and featured a faintly printed pattern surrounded by the stamp's value written in Swiss (which translates to three shillings in English). The regular issues were printed in green ink. However, a few were accidentally printed in yellow. The single surviving yellow Treskilling stamp was valued at 2.5 million Swiss Francs, or $2,788,000, in 2007.