What is the "ba" that 周恩来 used when speaking to Kissinger? Dr. Michael Pillsbury, Senior Fellow and Director, Center for Chinese Strategy, testified before the US Senate (p. 9) on Nixon's visit to China:

But there were glimpses even then that the Chinese saw the United States not as an ally but as an obstacle. Referring to the United States, Zhou offered a hint of how the Chinese really felt about their new prospective friend. “America is the ba,” Zhou told Kissinger’s interpreter, Ambassador Ji Zhaozhu of China’s Foreign Ministry, repeating a term that would be frequently used by Chairman Mao and his successor, Deng Xiaoping.
[...] Kissinger’s translator told Kissinger that Zhou’s statement meant, “America is the leader.” This seemed to be an innocuous remark, and when taken in the context of the Cold War even a compliment. But that is not what the word ba means in Mandarin — at least that is not its full context.

How would that statement have been constructed in Chinese, and to which &quot;ba&quot; was being referred to?
I guess it was 霸 /ba4/ : tyrant; bully; overlord  

Short for '霸主' (dominant leader among rulers)  or '霸權' (dominant power)

It was not a friendly word toward America, because it implied America was a bully, (try to dominate other countries of the world with it's strength)

In ancient time, the difference between 霸主 (leader among rulers) and 天下共主 (common leader of the lords) is a 霸主 didn't have the loyalty of the other rulers. They submitted to him out of fear. 

Since Zhou said "America is the ba" I presume he meant "美國是全世界的霸主" (America is the world's most dominant power)

美國是全世界的霸主

If Zhou said "America is a ba", then would mean "美國是一個霸權" (America is a hegemony)

Side note:

春秋五霸 (Five Hegemons) were five rulers who earned the title of 霸主 during the Spring and Autumn Period.