It's unjust that the most vivid image of Randy Johnson's career is the 1993 All-Star Game pitch he flung over the head of John Kruk to the backstop. The 6'10" Johnson came to the majors in 1988 as an archetype: a location-challenged power lefty who could throw the ball through a wall if only he could find the wall. He developed into a pitcher who combined power, command and a thoughtful approach to his craft. Now, having announced his retirement last week at age 46, he walks away as the greatest southpaw in history.
Any discussion of Johnson's transition from thrower to pitcher must include a reference to a consultation the Big Unit, then in the midst of a 12--14 season with the Mariners, had with Nolan Ryan in the summer of 1992. Ryan suggested a mechanical change—landing on the ball of his front foot instead of the heel—and the effects showed up almost immediately: Johnson cut his walk rate from nearly seven per nine innings to less than five over the rest of that season, and to less than four per nine innings in '93.
Johnson's development is the most remarkable thing about him, even more than the strikeouts (4,875, second all time) or Cy Young Awards (five, second all time) or wins (303). Johnson walked 152 batters in 1991, his second full season in the majors. He walked 151, total, from 2004 through '06. Calling Johnson the greatest southpaw ever is certain to rattle some cages, so think about this: Johnson had Sandy Koufax's career (2,324 1/3 innings with a 2.76 ERA) plus another nine full seasons of above-average pitching under conditions that were much more difficult than those Koufax faced. Johnson's four-year peak with the Diamondbacks (1999--2002) is also superior to what Koufax did from 1963--66 with the Dodgers, assuming you take into account the much more explosive offenses Johnson tossed against. Johnson had a harder job, and he did it with unmatched brilliance.