Scott Sumner concisely explains why Friedman is so esteemed:
During the 1960s, Friedman rejected 4 key tenets of Keynesian economics. And within less than a decade, all four of his critiques were shown to be correct. . . .
In one important respect, Friedman’s achievement is even more amazing than what I have outline here. In all four cases, Friedman’s claims were made at a time when they looked wrong.