First of all, no entertainer or sportsman should ever be considered for this. Its flat out rediculous to say someones entertainment or athletic talents were the most important contribution EVER given to this country. Total BS.
Now to get down to those worth considering:
Einstein - First off, not american (by birth) And while his knowledge was incredible, it wasnt so much a benefit to america but to mankind as a whole.
M.L.K. - Heres where I become unpopular. Yes, a great man. Maybe the greatest black american ever. He became the face and voice for millions of black americans who were standing up for themselves and their rights. However, black america is but one part of a WHOLE america. Im not suggesting this country is much better for his contributions but one segment of society benefited greatly, the other learned what all men are created equal truly means.
Inventors and humanitarians. Again, more for the benefit of mankind than just america. Sure we got the lightbulb FIRST but it changed the world more than it changed us.
Presidents - Clinton and Dubya? They will be lucky to be remembered as above average presidents. Only reason they are on the list is they are semi current.
Kennedy - Good president. Too short of a term. Cuban missle crisis was his shining moment. May have been one of the greatest had he had a full chance. Sadly, his legacy is more about promise and potential than actual deeds.
Founding fathers - Having studied a great deal of history Ive come to the opinion that while all of them were great men, none of them were truly VITAL to what took place. Of course there absence would be painfully missed but do you really think without a Washington that Frankin,Adams,Jefferson,Hancock,Madison,et all etc would have faultered? Ill admit G.W. was an incredible leader of men and one of our finest generals. I do think though that its rare one man can turn the tide of a war. Battles, yes. Wars no. That said, G.W. I love ya, but you had too much good help around you to give you most of the credit.
F.D.R. - As far as Im concerned, hes the greatest president ever. (looking at his full history) Yes, others dealt with bigger issues but he conquered many HUGE social problems, not just rescuing but improving the country at the same time. The depression and a World War are in his body of work. Programs such as the Tennesee Valley authority , the New Deal, Social security,and the FDIC all came to be in large part due to him. He is, without doubt or question, the greatest of the modern presidents. Without him, america could literally be a 3rd world country right now. Think about that.
And finally, Abe... While I think FDR is the most consitant and accomplished of all the presidents, Abe is the most important. America was completely divided and he somehow got us through a civil war and pulled us all back together. At the same time he laid the groundwork for freedom for ALL men with with E.Proc. So Abe is the greatest american. Not because he has the most contributions, but because he had 2 of the biggest and there would be no america anymore without him.