It depends. Washington was the man who could have been King of the United States. Yet, he chose to be an elected servant. On the other hand, there were other Presidents who would be King. Lincoln actually waged war against democratically elected governments and was responsible for radical changes in our government. Some believe it was a War he had to wage, and that the changes were unavoidable if the coun try were to be a viable republic. I'm sure many citizens would herald Abraham Lincoln as the greatest President. Franklin Delano Roosevelt steered the country through the Great Depression and World War 2. However you feel about the New Deal, his Presidency must be given respect. Doubtless, many would vote for him.
Unfortunately, John F Kennedy's name often arises in such conversations. After all, his Adminstration was heralded as "Camelot." In fact, he was no King Arthur. He won election by fraud and deceit. Both Illinois and Texas had rampant voter fraud, and Kennedy's campaign over a "missile gap" was a lie. The United States had a missile advantage over the USSR until the Cuban Missile Crisis convinced the Soviets that they must build up their nuclear forces so they would never have to back down again. Still, he had his moments, such as his visit to Berlin. Kennedy definitely was not one of the better Presidents.
Reagan pandered to victim's rights groups and signed into law a gun control bill which banned civilian ownership of automatic weapons, although they had not been a problem. he engaged the United States into Lebanon, then retreated after destruction of a barracks and massive loss of life. It was a show of weakness which inspired Islamists. Still, he had the backbone and vision to face down a declining Soviet Union, deploying Pershing Missiles in western Europe despite massive peace protests. His must be one of the better Presidencies.
Teddy Roosevelt was a pulp fiction President. What did he accomplish, other than to split the Republican Party and let Woodrow Wilson get elected? Wilson led the US into a War which otherwise would have stalemated into a brokered peace. The infamous Treaty of Versailles, which led to the rise of Nationalist Socialism in Germany, can be directly attributed to American intervention in WW1.
Harry Truman guided the country through the final phases of WW2. He was the man who said "the buck stops here," and indeed it did. He made the decision of whether to drop the atomic bomb on Japan, not once, but twice. He engaged communism in the Cold War with steely determination. Truman is one of my favorites.
Andrew Johnson is the Rodney Dangefield of Presidents. He had one hell of a tough job. The country was dominated by the Radical Republican, whose quest for vengence and conquest was not stilled by Appamatox. His efforts to quell retribution and heal the country deserve study.
My vote? George Washington, the President who steered the country down the road of democratically elected leaders. None of the others could have accomplished that. It's too bad we haven't followed his warnings about foreign alliances, etc.