This can be a President in your lifetime or before.
My vote is on Franklin Delano Roosevelt. Here are few of his accomplishments that changed America for the better.
Franklin D Roosevelt is still one the most remembered and honored presidents. He is remembered by many from his presidency that are still currently alive. He is also remembered from those who came after him through everything that history has taught. He is mainly remembered and honored due to the many accomplishments that he has made, these are his greatest accomplishments.
He created the FDIC, which is still used in our banking system today to ensure the security of individual accounts. Before the FDIC was formed many people were pulling their money out of the banks, due primarily to the fear that their money could at any time disappear. After the FDIC was formed people slowly began deposit money back into the banking system.
He also formed the Civil Works Program also known as the CWA. This placed many unemployed workers back into the workforce. The CWA was a public works program that built and repaired roads, parks, airports and many other needed structures.
FDR implemented the National Industrial Recovery Act, known as the NIRA. The NIRA did several things to improve working standards. One rather important thing that it created was the minimum wage that we still use to guarantee a set minimum hourly wage.
One accomplishment that he made while in office was created the Fair labor Standards Act. This Act set child labor laws. This eliminated the unfair exploitation of child workers.
FDR was also responsible for abolishing prohibition. This is often left out of his accomplishments, though I personally am very glad that I can have a drink from time to time when I feel like it without having to sneak in to one of those shady looking “speakeasies”. Not to mention that the sale of alcohol probably reinvented several jobs across the nation including manufacturing, distribution and service.
His single greatest accomplishment was nothing that can be measured by an analysis of his policies. What FDR did was restore American confidence and create sense of optimism in the midst of the Depression. He always sounded hopeful that things would get better and did his best to try and explain what he was doing to try and fix the problem through his fire-side chats over the radio. In doing this, many in America had great confidence in FDR and were highly supportive of him, which is partially why he remains one of the most popular presidents in American history.
In terms of policy success, his greatest real accomplishment was providing regulations for what business could do. The Great Depression was the result of several factors, including bad credit and greed on the part of banks. When the stock-market crashed in 1929, the financial policies that came out of the seeming blooming of the 20s were proved to be faulty, and FDR made sure that their mistakes could not be repeated. As a result, the US economy until 2008 after 8 years of Reagan, 4 years of HW Bush, and 8 years of W Bush (all of whom worked to undo FDR's policies) never suffered an economic crisis on the same scale as the Great Depression.
Other things like Tennessee Valley Authority, which brought electric power to the Appalachian Mountains and the Civilian Conservation Corps, which put people to work without drafting them into the military are also among his top accomplishments as President.
And in terms of foreign policy, his greatest accomplishment was being able to eventually rally America to the allied cause against both Nazi Germany and Imperial Japan. The Lend-Lease Act was FDR's main idea to supply Britain and other allied nations with the equipment needed to keep fighting. He was not, however, able to get America to actually take a firm stand against Japanese or Nazi aggression until after Dec 7, 1941 when Japan attacked Pearl Harbor and Hitler declared war on America a few days later.