Since February 1979, Gallup has asked Americans, "In general, are you satisfied or dissatisfied with the way things are going in the United States at this time?".

During that timespan, the highest percentage of "satisfied"s was 71% (26% dissatisfied), in February 1999. I don't know if there are earlier statistics for the period from 1945 to 1979.

For presidential approval, we're in luck: Gallup has been polling this ever since World War II. The highest ever Presidential approval rating was 90%, for George W. Bush immediately after the September 11th attacks. The highest average approval rating was 70.1%, for Kennedy.