George Washington gets vote #1 for all of history; Ronald Reagan #1 for modern history.

President Washington set the ground work for all other presidents; he could have been king--as some other presidents have and are trying to be--but he did not want autocratic power to run this country. Had the politicos of this country listened to Washington, many, many conflicts and wars could have been avoided because he strongly warned against foreign entanglements.

In President Reagan's favor is his spirit of the founders of the country as he quoted John Winthrop, who said in 1630 "For we must consider that we whall be as a city upon a hill..."  Assiduously, Reagan worked to restore nationalistic pride to Americans, to improve the economy and reduce the National Debt by cutting into the misuse of welfare programs and any superfluous expenditures, and to defeat the threats and evils of Communism.

We have actually had 43 men elected President of the United States.   Obama is the 44th, but Grover Cleveland was elected to two non-consecutive times, making him the 22nd and the 24th.

My vote would go to FDR.  His programs for social and economic reform helped pull us out of the Great Depression, and his leadership held us together through most of WWII.  He faced some of the darkest days in U.S. history, kept the country together, and was able to do all of this despite a difficult physical condition!  Plus, Eleanor Roosevelt--she is one of my favorite First Ladies.

I think the obvious answer to this is Franklin Roosevelt, who did more to change the war Americans think about the relationship of government and society than any other President. But I will go against the grain and argue that James K. Polk left the boldest mark on American history. He was unique in that he ran as a single term candidate, and in that he achieved much of what he set out to do. In particular, it was under Polk's leadership that the United States expanded to the west coast by virtue of victory in the Mexican War. I personally regard this war as a rather reprehensible act (I'm sure Reagan would've disagreed with me on that one) but you can't argue that it changed the nation forever, especially as it was a major stepping-stone towards disunion and the Civil War. Polk also went a long way toward establishing a modern Treasury system, in which federal funds were deposited not in private banks, but in a system of treasury offices. So, especially for a four-year president, Polk left an indelible mark on American history.

I would definitely go with Ronald Reagan. This is because, to me, the American psyche was quite strong and our nationalism was strongly identified during his time in the presidency. Of course, he was not perfect, but he has always been extremely memorable for the sense of pride that he instilled in his people.  The 80's displayed a lot of American pride and "Americanism" which, to me, is a reflection of a country which is, essentially, happy to be who they are. I do not see that American pride shinning any longer, partly because the current administration does not seem too keen on putting our country in the spotlight, except to apologize for it. 

I also agree wholeheartedly with #4 and also pick Teddy Roosevelt. 

I would have to go with Theodore Roosevelt as leaving the boldest mark on American History.  His vivacious personality, tough-guns approach to handling foreign affairs, and many public reforms made him highly popular with the American people. He is chiefly responsible for much of the conservation reform that took place across the country, and was responsible for the creation of Yellowstone National Park which led to the eventual formation of the National Park Service. He was reponsible for the building of the Panama Canal. He was also a notable author; his history of the War of 1812 is unequaled.   As a President, he led the fight for "trust busting," or the breaking down of large monopolies so that smaller companies could have their fair share of the market.  Today his face is familiar at Mount Rushmore National Memorial, we play with "teddy" bears, named after him because of his involvement with nature, and many movies have been made about his life and his exploits.

First of all, please note that it has not been 26 men.  President Obama is the 44th president.

I'll go with George Washington.  What Washington did that was so special was that he stepped down from power.  In a lot of other new countries, you have leaders like that holding on to power for life and setting a precedent for a completely undemocratic system.  Washington stepped down voluntarily, creating the expectation that all other presidents would step down when their time came as well.  This was hugely important for the success of our country.

This question is going to generate lots of answers. However, in my opinion, Barack Obama has and will leave the most important legacy. The reason for this is because race matters. 

Barak Obama is the first African-American president. This alone is an amazing fact, especially in view of America's past history of slavery and many generations of racism. The fact that we have an African American president shows that America is the land of opportunity. Furthermore, it shows that change is possible. 

It also says that this we are moving past some of the racism that has characterized us. Most importantly, this fact shows the way forward as well. One of the reasons why American became great was because it embraced all different types of people. If we are going to rise again, it will be through the same way.