Possibly Alexander Hamilton.
The libertarian love of a virtually non-existent federal government and a nation consisting of a patchwork of small state governments that magically function like a single nation is a pipe dream. Naive and impossible. The articles of confederation failed, and Jefferson was wrong about quite a lot.
Hamilton realized that in order for a nation likely to grow as vast as America has requires - despite the perils and drawbacks - a strong federal government and an energetic executive. The establishment of a national bank is one of the many contributions he made that proved absolutely vital to our growth as a single and strong nation.
We'd have been conquered in short order if we adopted Jeffersonian/libertarian views of government.
(Of course, we wouldn't have gotten far had it not been for Lincoln's fight to preserve the union. I rank Hamilton before Lincoln because I'm not entirely convinced that Lincoln was the only politician who would have seen the necessity of fighting and winning the civil war. I suspect the war would have been fought without him, and won. Slavery, eventually, would disappear as society progressed.
Hamilton's contribution was an absolute necessity, and he put inhuman effort into making it. Lesser men would have failed).
But, of course, there are any number of persons or events in American history which, if they did not exist or if they played out differently, would have lead to a drastically different reality. So it's impossible to say anyone is the "most important".
As has been pointed out, if someone other than Ben Franklin went to France, it is possible that France would not have provided crucial assistance and we would have been conquered by England. Etc.