JFK's overrated in that factor. The German Reich was on the edge anyway.
I thought Bobby Kennedy was a good leader. Many may have shouted about his running after Jack being an attempt at a royal "Kennedy dynasty", but he did very well in a post Nazi world and played a key role in organizing the world after the Nazi empire fell.
However, President's Franklin Roosevelt (1933-1945) and Henry Wallace (1945-1949) did lead us to victory over the Japs in the Pacific war which was the pivotal point of the US in the 20th century.
OOC: This is in After 1900, so I'm keeping it to the 20th and 21st centuries.
Reaganomics was a very bad economic decision. A quick boom followed by massive deficit, class inequality, corporate corruption, and a Second Gilded Age crashing into economic turmoil which we are trying to resolve today.