Barack Obama has been ranked as the 15th best US president, just below Bill Clinton but ahead of Ronald Reagan, in a new poll of leading presidential scholars. The Siena College poll, which surveyed 238 presidential scholars at US colleges and universities, rated Obama two places below Clinton, who was 13th best, and three better than Reagan, who is ranked as the 18th best. Franklin D. Roosevelt again earned the top spot, as he has every time since the poll was first conducted in 1982. He and the Mount Rushmore presidents - Teddy Roosevelt, Abraham Lincoln, George Washington and Thomas Jefferson - have consistently been the top five presidents in the poll's findings. Obama's 15th ranking is slightly higher than other presidents who have taken office since the poll started nearly 30 years ago. Most start out at about number 20, said Siena statistics professor and poll director Douglas Lonnstrom. "[Obama's] doing a little better, but he's generally in the same ballpark," he said. While he ranked high on traits like imagination (6th), communication ability (7th) and intelligence (8th), Obama rated poorly ratings on background (32nd), which was composed of traits like family, education and experience. Obama's predecessor, George W. Bush, was ranked at number 23 in 2002 - the last time Siena's presidential expert poll was conducted - but has since dropped to number 39, qualifying him as one of the five worst presidents. Bush came in at number 42 - second to last - on issues such as handling the US economy, foreign policy accomplishments and intelligence. Warren G. Harding was rated the least intelligent president. Bush joins Harding, Andrew Johnson, James Buchanan and Franklin Pierce, all of whom have consistently ranked as the worst presidents since the poll started, in the bottom five. Several other presidents also saw movement in their ratings this year. Bill Clinton moved up five places, from No. 18 in 2002 to No. 13 today; John F. Kennedy also moved up, from No. 14 to No. 11. Carter, Reagan and Nixon all dropped in the rankings this year - Carter dropped seven spots, from No. 25 in 2002 to No. 32 now; Reagan dropped two spots, from No. 16 to No. 18; and Nixon fell four spots, from No. 26 to No. 30.