This is a good question, but a hard one to answer. As
@mrstime says for most Americans George Washington comes to mind, without his leadership and sacrifice the US wouldn't be what it is today. Hard to imagine what it would be but probably less than it is now. The problems with Washington include his slave ownership and mistreatment of Native Americans. But many of our greatest women and men had flaws, particularly when judged by today's standards.
@CallMeKate is right about that.
I like
@Mike 's suggestion of Winston Churchill, without him WWII would likely have had a very different outcome. He and Stalin were probably the two most important people in the allies victory, and Stalin certainly doesn't make the list, not even close. Roosevelt was also important, but in my opinion less so than Churchill.
Three not yet mentioned that come to mind would have to be Juan Carlos I, Abraham Lincoln, and Mother Teresa. I think Juan Carlos did a very impressive job of transiting Spain peacefully from the fascist Franco era to a democracy. Some parallels to Washington, Juan Carlos took the title of King, but rather than make it powerful he turned it to more of a ceremonial thing, like the UK. He could have taken a lot more power. However I don't really know a lot about him, I do know that of late he's had his problems.
Mother Teresa is someone I don't know a lot about, but what I do know is she gave her life to help the poor and sick. She also managed to inspire a lot of other people to do similar things. But like other heroes she wasn't without fault, no real ones are. To quote
Wikipedia: "
Mother Teresa was admired by many for her charitable work, but was criticised for her views on abortion and contraception, as well as the poor conditions in her houses for the dying." None the less she is high on my list.
Abraham Lincoln, I believe, deserves much credit for making the US what it is today. Without his leadership and wisdom we would not likely be one country, and the south might still be practicing legal racism. And he seemed to do it without much personal gain.
My list could go on, I missed some good candidates like Martin Luther King and Thomas Jefferson...
If forced to chose I might agree with
@Old Salt and go with Nelson Mandela.
