What is the "ba" that 周恩来 used when speaking to Kissinger?
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.