To be fair, if you want to look at it from the perspective of ERA+, which isn't entirely unreasonable, Johnson is the clear winner. Koufax had 8 seasons in which he qualified for the ERA title. Compared to Johnson's 8 best seasons, you can pick any sample size of best years you want and Johnson comes out ahead:
Johnson |
Koufax |
Season |
Age |
ERA+ |
Season |
Age |
ERA+ |
1997 |
33 |
197 |
1966 |
30 |
190 |
2002 |
38 |
195 |
1964 |
28 |
186 |
1995 |
31 |
193 |
1965 |
29 |
160 |
2001 |
37 |
188 |
1963 |
27 |
159 |
1999 |
35 |
184 |
1962 |
26 |
143 |
2000 |
36 |
181 |
1961 |
25 |
122 |
2004 |
40 |
176 |
1960 |
24 |
101 |
1994 |
30 |
152 |
1958 |
22 |
93 |
That being said, as I've said in the past, even + stats can't necessarily do an adequate job of normalizing eras. Johnson played in one of the most offensive eras in league history. That means the numbers are bigger, and the variance is bigger, so it's easier to have slightly more impressive + stats, especially as a pitcher. Koufax played in the least offensive period of the live-ball era, making it very difficult to achieve the same ERA+ numbers. I mean heck, his best ERA seasons were 1.73 and 1.74. It's hard to be much better than that; anybody human is going to make mistakes, and in live-ball baseball a few mistakes add up to a high-1s ERA at best. Since 1920 only 8 pitchers have combined to put up 9 seasons with better ERAs than what Koufax had in '64 and '66. Probably Hubbell doing it in 1933 is more impressive, or Maddux in '94 and '95, or Kershaw being only slightly worse the past few seasons. But still, those numbers are so far to the high end, I think they might merit a little more appreciation than the + stats would imply, because of the run scoring environment in which they occurred.