It's been 13 years since NASA's Opportunity rover is exploring Mars. In your oppinion what is it's most important discovery to date? Is it our most succesful Mars rover? Will the next Mars rover (planned for touchdown on Mars surface in 2020) have to chance to achieve even more? What would you personally like to see as it's scientific payload?
Andrew Rader (SpaceX engineer, MIT PhD, author)

Perhaps the greatest discovery of the Spirit and Opportunity rovers has been to study the water cycle on Mars and yield clues as to how ice and frost moves about the planet with seasons and weather, although it would be hard to argue that Opportunity's greatest achievement isn't its marathon longevity. Curiosity and the 2020 Rover are much more capable than Opportunity so should interact more with the planet and (presuming a long mission) may even eventually travel farther. I think the most important experiments going forward are related to the search for water and life on Mars, and starting to conduct experiments on use and conversion of local resources like the production of methane, oxygen, and liquid water.

Nancy Atkinson (Senior Editor for Universe Today, Host of the NASA Lunar Science Institute podcast & a NASA/JPL Solar System Ambassador)

I think its interesting that basically everywhere Curiosity has traveled, it is finding evidence of past water. From the rounded pebbles that were worn by flowing water to the mudstone and sandstone features, to the layered rock formations that could only be laid down in large amounts of water, it appears that Gale Crater was at one time filled with water. And that's intriguing because we know on Earth, everywhere there is water, there is life. Curiosity has been finding these features and potentially habitable environments almost since it landed, so the choice of Gale Crater as the landing site appears to have been the perfect place to explore!

I'm really looking forward to the Mars 2020 rover, especially how it should be able to test ways for future human explorers to use the resources available on Mars to ‘live off the land.’ Also, it should help us understand the hazards posed by Martian dust and demonstrating technologies to process carbon dioxide from the atmosphere to produce oxygen, which could be used for the production of fuel. 

Antonio Paris (Astronaut Candidate, Astronomy Professor, Planetary Scientist, Space Science Author)

The Mars rover Opportunity has made many groundbreaking achievements in the exploration on Mars. Its greatest achievement, in my opinion, does categorically fall into science or technology. I believe that Opportunity’s greatest achievement is that it served as an “extension” to the human eye, thus allowing us to explore a far distant world where humans are still decades away from making landfall. Additionally, none of the rovers on Mars are more successfully than the other. Each robotic mission to Mars had a specific purpose and it was their cumulative discoveries that have made the exploration of Mars a success thus far. Moving forward, there is an assortment of Mars rovers that will one day take the helm for Opportunity. As technology continues to improve, I sure hope a HD live cam makes it way into the next rover’s payload!