Popularity ratings are a fairly modern phenomenon. The first, and the one that is most often cited, is the Gallup poll, which only began tracking presidential popularity in the late 30s with the Roosevelt administration.

Gallup now provides an overall average, term averages, and max/min ratings for all presidents between Truman and Obama.

Based on these figures, Truman was the least popular president with an average rating of 45.4, marginally shading Carter (45.5, meaning 45.5% of respondents approved of his handling of the job). Kennedy (70.1) had the highest approval rating. Trump, of course, does not have a presidential rating yet, but for comparison Gallup does track his approval rating as PEOTUS handling the transition. His rating as of 13 January was 44, compared to an 83 for Obama at the same stage in his preparation for the presidency, with a margin for error of +/- 4%.

Since this is AskHistorians, it's interesting to contrast these strictly contemporary figures with the occasional polls conducted among historians – for example the Schlesinger poll and the Wall Street Journal poll – which consistently place Truman among the top 10 presidents of all time, on several occasions rating him as high as fifth.

James Buchanan, Andrew Johnson, Franklin Pierce and Warren G. Harding are the presidents who feature in the last three places most frequently in these surveys of historians.