There are three main reasons why President Franklin D. Roosevelt is seen as a great president while President Herbert Hoover is seen as a very poor one.
First, and probably least importantly, most historians are relatively liberal. FDR was a liberal icon because he expanded the scope of what the government was willing and able to do to help improve the American economy. Hoover represents a much more conservative approach to government and the economy and so liberal historians are much more likely to think that FDR was a good president.
Second, FDR was much better at handling the Great Depression. Hoover gets a bad rap because people think that he did nothing to try to end the Depression. In reality, he expanded the role of government (compared to what previous presidents had done), but he did not expand it enough for such a tremendous emergency. Perhaps even more importantly, he did not show that he understood the psychological needs of the American people at that time. Hoover did not show the sympathy that Roosevelt did and he did not communicate in such a way as to reassure the people the way that FDR did in his “fireside chats.” In other words, FDR did a good job of changing government policy (better than Hoover), but he was really great at making the people feel like change was coming and that they could have a reason to be hopeful. Hoover, by contrast, is seen as having been in denial about the Depression and as having a “tin ear” about what to say to the people about it. This was extremely important and is a major reason to see FDR as much better president than Hoover.
Finally, FDR is not only defined by the Great Depression. He might be seen as a great president just because of that, but he was also the country’s leader for most of World War II. Roosevelt’s leadership in the war was exemplary. He did a good job of setting strategy for the war and he did a tremendous job of dealing with a set of difficult allies in the UK and the USSR. As the man who led us through one of the most horrible wars in our history, FDR gets a great deal of credit from historians. Hoover had no such chance to be seen as a great war leader.
These are three of the main reasons why FDR is seen as a great president and Hoover is not.
In any such statement of "ranking" or "good/bad" comparison, the enquirer must first ask what the criteria are for the comparison. What qualities in a president make him (and some day her) good or bad? Honesty? World reputation? Effectiveness? etc. Roosevelt held the office in a time when the nation was having many problems -- financial, international, etc. But Hoover, whose reign was much more tranquil, may have been more honorable, more compassionate, a better compromiser, a closer representative of his constituency, etc. The fact that Hoover did not have a defining moment, a signature problem to solve, does not automatically make him a lesser president, just as Polk or Taylor were not worse than Lincoln or Eisenhower. My point is that comparisons are odious; an essayist must clearly define the measuring scale.