It all depends on the criteria. If pressed, I'd say either Lincoln or Franklin Roosevelt because they overcame staggering adversity. For effectiveness (in a rather ignoble cause) I'd say James K. Polk, who fulfilled his campaign promises and then some by adding the Mexican Cession. It should go without saying that Washington should be up there too if for no other reason that he established a policy of neutrality that was upheld until 1917. He was not exactly as highly regarded in his own time as his mythology would suggest, however, and his presidency really did amount to a counter-revolution for many common people.
I agree with those who have mentioned George Washington. Abraham Lincoln would be a close second. (These guys are not on Mt. Rushmore for nothing.) I am always struck and moved by something Lincoln said (it's quoted in Aaron Copland's beautiful "Lincoln Portrait"): "As I would not be a slave, so I would not be a master." Imagine what our country might be like today if slavery had never happened.
George Washington was certainly a great man and did much to make the U.S. a success; however one should also consider Lyndon Baines Johnson, who had considerable experience as a legislator and brought that experience to the White House. Under President Johnson, despite the ultimate failure of the War in Viet Nam, more substantive legislation was passed than under any other President, including the Voting Rights Act of 1965, the Civil Rights Act of 1964, and the institution of Medicare, among other programs.
I completely agree with the first part of the previous post. Washington is the greatest because he stepped down from power. This is very rare among people who take power in a new country. Usually, you get people like Robert Mugabe, trying to hold power forever. If that's what we had had, our history would be so much different...
I can tell you that the worst is the German president... but the best I'm not sure about Gordon Brown who was Prime Minister of England was the best in my opinion of course I'm not American so I can't help...
And from outside the United States, "Sultans of the Ottoman Empire" not all the sultans, of course.
You can see a full account of these polls at the site below:
Historical rankings of Presidents of the United States
I can't comment on the best 'President', as I am not American. However, I will comment on the most effective Prime Minister of Australia. I believe John Howard was, as he got things done and wasn't hesitant to make decisions.
Maybe Ronald Reagan or Warren Harding because they did nothing to stimulate the economy during the shortest possible recessions.
My great grandmother hates Woodrow Wilson because he sent men to war without guns, but his ideas about national boundries in Europe were visionary, so he could get points for that.
Dwight Eisenhower and George Washington because they were the two presidents who didn't want to be president.
Henry Henry Harrison and Thomas Dewey for being president for the shortest possible time.