The Mars Pathfinder has got to be in the running....

Released: Dec. 10, 1997.
Mars Pathfinder landed in the Ares Vallis region of Mars on July 4, 1997.
The Mars Pathfinder stamp features the lander and the Sojourner rover as it prepares to explore Mars. Credit: NASA/JPL
Pathfinder's deployed a rover, Sojourner, to conduct detailed analysis of the terrain surrounding the landing site. While Sojourner traveled only 0.4 inches per second, the little rover that could remained hard at work on the surface of Mars for nearly three months, far surpassing its design life.
From landing until the final data transmission on September 27, 1997, Mars Pathfinder returned 2.3 billion bits of information, including more than 16,500 images from the lander and 550 images from the rover, as well as more than 15 chemical analyses of rocks and soil and extensive data on winds and other weather factors.
Findings from the investigations carried out by scientific instruments on both the lander and the rover suggest that Mars was at one time in its past warm and wet, with water existing in its liquid state and a thicker atmosphere.
Did you know -- Remote control commands from Earth took about 10 minutes to reach Sojourner!
This stamp featured an image that was beamed back from Pathfinder showing Sojourner as it prepared to explore the Martian landscape.