That's a tough one! The United States intervened in foreign affairs during all of those periods; however, I would say that the majority of US intervention occurred during the Cold War.
During the Cold War, the United States overtly and covertly impacted the political make-up of numerous foreign nations. For instance, during the 50s, the US led the United Nations in fighting North Korea after its leader, Kim Il-Sung, invaded South Korea. Similarly, during the 60s and 70s, the US waged war against North Vietnam in an effort to prevent Ho Chi Minh from uniting the country as a communist nation.
Those are some overt examples, but the United States also partook in more subtle interventions as well. The Marshall Plan which funded European reconstruction following WWII could be seen as an economic intervention as its goal was to closely align Western and Central Europe with the United States rather than the Soviet Union. Similarly, the US funded a number of assassinations and coups. For example, in 1953 the US backed a coup against the Prime Minister of Iran, Mohammad Mosaddegh, after he began to nationalize the oil industry there. A similar coup was attempted against Fidel Castro in Cuba during the Kennedy Administration with the failed Bay of Pigs Invasion in 1961.
So I would argue that the Cold War saw the bulk of US intervention in foreign affairs. While the World Wars seem like clear examples (due to the highly-publicized nature of foreign war), the United States's efforts to roll back and oppose the spread of communism during the latter half of the 20th century trumps any single war regarding the amount of intervention involved.
