Who *are* the greatest living Americans? It's a hard question, for a number of reasons.
Firstly, "call no man lucky until he is dead", similarly, trust no evaluations of greatness until their accomplishments have stood the test of time and weathered a crisis or three. Five years ago, Alan Greenspan would have been on my list. Three and a half years ago, Ben Bernanke would have been on it. At one point George Soros was a candidate, before I read more about him & realized just how much of a villain with good publicity he really was.
Secondly, don't confuse good intentions with greatness. George W. Bush is a good man, and occasionally a competent politician and passable statesman. He still failed when it was important, and left a mess for those that come after him. The same goes for most politicians, whom as a class are generally second-rate intellects and second-rate human beings, at best.
Thirdly, it really does seem as if fewer and fewer real innovators, real leaders, real thinkers are left in our polity. The true giants were by and large pre-boomers, and are dying off at a rapid pace. Milton Friedman is gone, so is Norman Borlaug, Ronald Reagan, Sam Walton... The heroic figure has largely been eclipsed by the celebrity in the public eye. Despite all the self-serving Randian piffle of the last two years, there are no John Galts or Howard Roarks on the national scene; the best we can offer is Donald Trump. We don't really have any true robber-barons, and many of those we have, aren't Americans at all, like Rupert Murdoch or Richard Branson.
Fourthly, there may be a set of "greatest living Americans" whom no-one has ever heard of, "for the growing good of the world is partly dependent on unhistoric acts, and that things are not so ill with you and me as they might have been is half owing to the number who lived faithfully a hidden life and rest in unvisited tombs." Our awful, worthless media are of no help in this regard.
I don't know, I'd essay Rudy Guiliani, if only for how he did the one thing that no-one would have ever expected in the days of my childhood - redeeming unredeemable Gotham, diverting it from its self-demolition, if only for a generation, if only for a bit... Perhaps T. Boone Pickens? It's sometimes hard to tell a true captain of industry from a self-promoting con artist of monumental proportions. Look at Lee Iaccoca!
Logically speaking, there should be some great man or woman lurking in the medical research field, with all the activity of the last thirty years, but it's all been so bureaucratized and corporatized, it's hard to spot the motive man among the Organization Men... I don't know, maybe Leland Hartwell or Robert Horvitz.
(While looking around, I noticed this strange duck, Ahmed H. Zewail. Can't decide if he's technically an American or an Egyptian. He seems to live in the US, with citizenship, but anyone who has rumors about him running for the presidency of another nation is a dubious example of an "American". It's like claiming Golda Meir as an American, I think.)
Nanotechnology would seem to be a fertile field for earth-shakers, but the only people in that field I'm likely to have heard of - like Eric Drexler - are most likely nothing more than popularizers. Huh, looking around, some of the American candidates in nanotechnology are already dead - Richard Smalley for one. The field hasn't even come close to fulfilling even a hint of its promise, and already the pioneers are leaving us. Yet another example of how greatness is only proven in many cases posthumously. Examples like Norman Borlaug are rare indeed!