Atually - upon further thought, Neil Armstrong should be called the greatest american. But if not - its no big deal. He'll end up being the one remembered the longest.
You know, in ten thousand years people will not remember Bill Gates, or George Bush, or Madonna. Or John Wayne. Or Queen Elizabeth. Or any sports star you ever heard of. Or entertainer. Or General. Or Hitler. Or Ghandi. Or Martin Luther King. Or George Washington. They will have forgotten WWII. WWI. The Cold War. All the wars of the second millenium. Mohamed. They very likely will have forgotten what a United States of America was.
But they will look back from wherever it is they're sitting on in whatever parts of the universe they're calling home at that time, and they'll remember the name of the first of humanity's children to step foot on some body other than their birth planet. Neil Armstrong.