While all three periods you mentioned brought significant changes to American life, the post-World War II era (1945-1968) arguably produced the most substantial change across the broadest spectrum of American life. Here’s why:

• Politics: The rise of the two-party system with strong Democrats and Republicans, the end of isolationism and the beginning of the Cold War as a defining foreign policy concern.
• Economy: The unprecedented economic boom fueled by wartime mobilization and the G.I. Bill, leading to a rise in the middle class and suburbanization.
• Diplomacy: The formation of the United Nations and the United States’ emergence as a global superpower.
• Society: The Civil Rights Movement dismantling segregation and Jim Crow laws, the rise of feminism and the Second Wave of feminism, and the counterculture movement challenging social norms.
• Culture: The rise of television as a dominant media force, the growth of consumerism, and the emergence of rock and roll music as a cultural phenomenon.
• Gender and Race: While significant challenges remained, this era saw a push for women’s suffrage (achieved in 1920) and the beginnings of major advancements in civil rights for African Americans. The Civil Rights Act of 1964 and the Voting Rights Act of 1965 were landmark achievements.

The Progressive Era and the New Deal were undoubtedly transformative periods as well.

• The Progressive Era addressed issues like monopolies, unsafe working conditions, and political corruption.
• The New Deal introduced social safety net programs and government intervention to stimulate the economy during the Great Depression.

However, the changes of the post-WWII era were arguably more sweeping and long-lasting. They fundamentally altered the social fabric, economic landscape, and global role of the United States. It was a period of rapid economic growth, social upheaval, and the questioning of traditional values, all of which continue to shape American life today.

It’s important to note that historical change is complex and periodization is subjective. This is just one perspective on the most transformative period in American history since 1877.