In my humble opinion;
- Theodore Roosevelt. He started the National Parks system. He started the minimum quality standards for food and medicine that we largely take for granted today. He started the policies of government providing public sanitation services (e.g. garbage and sewer) to populations that could not otherwise afford it.
- Franklin Roosevelt. He helped to end the Great Depression. He led the US through the bulk of WWII. He was the only president to ever serve more than 2 terms.
- Ronald Reagan. He faced down the Soviet Union and ended the Cold War. He ended the redundancies from Roosevelt's New Deal. He's the only president to survive an assassination attempt.
There are plenty of other's to choose from: Hoover, Bush Sr., Clinton...but Teddy, FDR, and Reagan get my vote.