When you say best, what do you mean? Each president has had to deal with different circumstances during their terms, which in essence could've hindered the way they were viewed. Sort of the way Harry Truman was viewed during his term, he didn't get a lot of the credit he derserved while he was in office, and actually left the White House with little fanfare, but history viewed him differently as time went on, and showed him as a great man, and leader, albeit when he passed on.
Then you have president's that stepped into a good situation, see Ronald Reagan. Through the ongoing talks between Jimmy Carter, and Jesse Jackson, the hostages were freed from Iran, but Carter's term ended before the hostages were freed, and history went on to show Reagan as the man who freed them, while showing Carter as a failure. I was glad to see him recieve the Nobel Peace Prize, as vendication.
All in all, I would look at the circumstances, and say that FDR would be the greatest president, if not for the 3 consecutive terms he served, but the way the country pulled together for him during WWII, at the same time coming out of the Depression.
NICE