For me, his truly legendary status is earned by his suppressing the Newburgh Conspiracy near the end of the war, demobilizing his army after the end, turning down suggestions of a throne and standing down after two terms as president. There were a lot of opportunities for the US to start life as a banana republic.
When George III heard that Washington had refused "the throne" and stood down after his time as President, he said what an admirable man GW obviously was. It's a shame in many ways that they never met, as they would have had a lot to talk about, both being keen farmers with a strong sense of duty. Actually, they both also seem to have been rather boring and laconic in private, so whilst they might have had a lot to talk about, they probably wouldn't have said very much!