The problem with ranking the US presidents is that the ones of recent memory often do not yet have enough post-office historical analysis and are instead judged on popularity. That being said, here is my list:
1. Abraham Lincoln: If you've read his biographies, you'll see that he was possibly the most moral person to serve in the office inspite of mutiple personal difficulties like manic depression and Murfan's (sp?) syndrome. His Emancipation Proclimation and his inspiring delegation of authority to the Union generals during the hardest phase on US history are the great things of like may never happen again.
2. George Washington: His two terms in office helped keep the country together even though there was a great deal of dissent among those who helped in the battle of independence. Perhaps Washington's greatest exploits came in the fourteen years (1775-1789) before he took office; it was that long before the US constitution was adopted and it may never have come about without old George.
3. Harry Truman: Undoubtably the best president who first took office unelected. He was an honest, straight talking man who had no concern for his own glory, wealth, or popularity. He showed great courage in his used of atomic weapons to end the war with Imperial Japan and perhaps greater courage in firing Douglas MacArthur (extremely popular at the time) for insubordination.
4. Ronald Reagan: He was exactly what the country and the world needed in 1981. The US was suffering from a very poor economy, federal intervention was nearly out of control, no real let-up from the malaise declared by his predecessor, and a general lack of respect from allies and adversaries abroad. Congress had been improperly using its power over the country due to the weakened state of the executive branch since Watergate seven years earlier. The first six years of his presidency were outstanding in what he achieved; alas, the last two years were not so bright as he then had failures due to overconfidence in some of his underlings: not just North and Poindexter, but VP George Bush as well as the head of the National Security Council.
5. Franklin Roosevelt: He was great, but not for his plethora of depression era programs that barely passed constitutional muster (some didn't), but rather for his leadership in World War II.
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And second from last:
41: Bill Clinton: Perhaps the sleaziest character to hold the office based on his personal conduct. Books and books have been written about his misconduct (and his wife's) before and during his administration. The only elected president ever to be impeached, his pardons from his last day in office would, in a just world, be enough to put him in jail for the rest of his life.
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And the worst:
42: Richard Nixon: His arrogance and his subversion of the constitution were so extensive that neither is matched by any other president.