He attempted to advance human knowledge and capability peacefully with hope of great things. Carl was an extremely good astronomer, astrobiologist, astrophysicist, and additionally an immensely good advocate when it came to the abolishment of nuclear weapons. He was the greatest man at that and the greatest Carl Sagan I could possibly conceive.
How do we define "the greatest man" let alone a great man? Who should we get consensus from? What criteria will we use? Since we will not be able to come to an agreement on these questions we will not be able to answer that question or to claim that he is or is not the greatest man.
I am not saying Carl Sagan is unimportant or "not great." To claim he is the greatest would be a slight to all those who have come before him. First, we need to define what "great" is. If we are to look at its translation from the Latin word "magnus, a, um," we see "great" should be used to designate size. If so, Henry VIII of England is a better candidate than Sagan. If we are looking at scientific impact, a far more appropriate comparison, Isaac Newton's expansion of mathematics with calculus and physics with gravitational theory and motion laws place him on a higher plane. If we are looking at impact on the global scale, Genghis Khan conquered more land. Unless we have a clear definition of great, no man or woman can claim to be the greatest.
It would have to be someone other than Carl Sagan. No disrespect intended, but Mr. Sagan just hasn't earned that accolade. Granted, the man was a smart scientist. Still, he wasn't Einstein. There have been many others, in Science, who are held in higher esteem for their discoveries & opinions. Sagan's theory of "Nuclear Winter", introduced back in the early 80's, is interesting. But it's never been proven (thankfully). So, even in the realm of scientific theory, this fails to elevate his status.
Carl would object to any person saying he was the greatest man to ever live. Mr. Sagan believed that life was great, and Science was a good way to understand it. Carl brought Science to the forefront of public attention in a time where many people lived in fear of one of it's products. His message of cultural tolerance, world peace, and respect for Nature were pivotal in changing the closed minded agendas of pro-nuclear politics. He inspired the young people of many generations, including mine, to seek the truth in their own fashion despite current ideas because that was what Science is about. Science is about being happily surprised when you are proved wrong, because that means you are one step closer to the truth. Only where there is freedom of ideas can progress truly survive. The best answers often come from the strangest places. Even though we only shared 8 months on this pale blue dot together before he passed, he managed to leave a legacy that I was able to learn from. For that, I will always respect Mr. Carl Sagan. I know that there will be more people to come who will join the ranks of Carl Sagan.