Without Washington, the entire nation would look different, probably unimaginably so.
There would either have been a civil war much earlier (and with a better chance of succeeding), or someone would have seized power, or there would have been a foreign takeover.
We would have been Mexico, basically.
What Washington did (especially what he DIDN'T do) in controlling the power given to him, and in listening to and mediating between the opposing forces within the government set the stage for the rest of American history.
Lincoln probably comes second for his willingness to sacrifice everything for the idea that a divided United States (what an oxymoron) would destroy everything that was built by the founders and the subsequent generations. And if he had lived long enough to put his plans in place for rebuilding the South, he might deserve to be number one (John Wilkes Booth might be to blame for an awful lot of the political situation in the US in the ensuing century-and-a-half, especially when it comes to what happened in the South).
Once you get to number three, that is when it gets difficult. I could go with Dr. Martin Luther King, Jr., because of what he both did and tried to do in order to create true equality in a nation built upon that idea, or what up until that time was more of a joke.
Beyond that, there aren't too many famous (for lack of a better term) people who had enough of a direct effect to stand out from others like themselves.
If you want to go small scale, you could throw in
Joshua Lawrence Chamberlain, who was instrumental in the Union victory at Gettysburg by holding the flank of the line around Little Round Top, one of the more famous "minor" moments in American history that could have caused huge changes if altered.