Originally Posted by Ustwo

A big part of the issue is what makes someone 'Great'.

Were they a Great American because without them America would have failed as a nation?
or were they are Great American because they embodied the American Ideal?

For the first, it would have to be Abraham Lincoln. While any of the founding fathers also fall into this category, Lincoln faced the first real challenge where if he failed the US would cease to exist as the US.

For the second, it would be Teddy Roosevelt. He was everything an American should be, his life couldn't have been written any more 'American' then it was.