CAN A WINE EVER be perfect? If so, who is qualified to pass that judgment? Is it the winemaker who is trained in viticulture or oenology? The merchant with a fabled palate who buys the wines year after year? Or is it the critic, with no formal training but a strong sense of smell, a notebook and an ability to taste 10,000 wines a year?

In the early 1970s a young lawyer from Maryland named Robert M. Parker Jr. was convinced it was the last of these—and devised a system that set out to prove it.

...