You probably have a decent, but somewhat vague, idea of exactly what the Big Train accomplished in baseball. You can look through his career numbers, but it’s worth spelling out some of his more remarkable stats to highlight his uniqueness and some of the best arguments his backers can make for declaring him the greatest pitcher ever.
Johnson threw 110 shutouts, including 8 years with 7 or more shutouts. He had 12 years of 20 or more wins, including 69 wins in 1912 and 1913 combined. His 1.59 ERA for the decade of the 1910s, when he averaged 7 shutouts a year, was good for a cumulative ERA+ of 183, which, as his Baseball-reference Bullpen entry notes, would rank as ten of the top 100 single seasons for ERA+ if you divided it into 10 individual years. In the 1910s, Johnson had an average of 343 innings pitched per year. And, he recorded 20 saves. (He finished in the top 10 in the A.L. for saves 11 times.)
He was first in the A.L. in strikeouts 12 times. His 2.17 career ERA is first among starting pitchers who spent a sizable amount of their careers after the deadball era ended, with Pete Alexander’s 2.56 the next closest among starters.
Johnson had 5914 1/3rd innings pitched, the equivalent of 30 seasons at 200 innings a year, which strikes me as similar to thinking of how matching Rickey Henderson’s 1406 steals would require 50 steals a year for 28 years. His two years of >300 strikeouts, in 1910 and 1912, were the last in MLB until Bob Feller came along, and he was the last to do it twice until Sam McDowell and Sandy Koufax in the mid-1960s.
As for hitting, the Big Train hit at least .270 four times in the 1920s, peaking at .433(!) in 1925, when he went 42-97, slugging .577, with an OPS+ of 163. That, I would guess, is a record for starting pitchers, or at least a record for starters older than 35. Then, in 1927, at age 39, he hit .348 over 46 at-bats. He slugged above .400 five times, twice in the 1910s.
Johnson’s career spanned 1907 through 1927: in that time, the Senators’ cumulative winning percentage was .492, quite a bit better than I would have guessed. The Senators won .506% of their games during the 1910s, his prime, and won 90+ games 4 times in the Big Train’s career. But, they were .325% in his rookie 1907 season, and .276% in 1909, which are probably the two worst records posted by a team with a great, great player who was playing relatively well. The 1935 Braves, at 38-115, had Babe Ruth for a couple months, but he hit .181 in 72 at-bats.