Which leader was most closely associated with stopping the spread of communism?

The answer is President Harry Truman

Harry Truman was the 33rd President of the United States serving two terms between 1945-1953

As the first post-World War II president, President Truman launched massive campaigns to stop the spread of communism from China and Russia and develop a completely parallel worldview.

He launched the successful Marshall Plan to rebuild western Europe, established NATO and the Truman Doctrine and intervened in the Korean War.

By rebuilding Europe, develop the military alliance of NATO and establishing Truman Doctrine, he was able to influence the next 2 decades of American policy to counter the growth of communism.