20th century and others tally dates from the first president.
Polls were weighted based on numbers of polls. However, this is not a meta-analysis with various individuals collapsed into the numerator and divided by the standard deviation in the denominator.. Rather, it looks at the mean rank (assuming hard number theory) of 32 polls based on averages.
Time is especially important. After President Ford died, his ranking quadrupled. Reagan's rank has improved dramatically. George W. Bush's standing has gone downward and Harry Truman and Dwight Eisenhower continue to climb. At the time of this writing, President Bill Clinton is the highest ranking ex-president.
RESULTS
The rankings at this time are the following from 32 polls going back roughly ten years.