I ran this down years ago using the same method for a career, Lefty Grove came out #1.
To commemorate the opening week of the baseball season and a slow day for me, I decided to come up with a top 10 for single seasons.
My method was simple. The pitchers had to throw at least 200 innings. The rest was league average ERA the season I'm analyzing, divided by the pitcher I'm analyzing's ERA for the same year. It's the only way I can think of to get a true apples to apples comparison.
Using my succinct method, here are the results.
1.Greg Maddux 1994 - 271% below league average.
2.Bob Gibson 1968 - 267% below league average.
3.Walter Johnson 1913 - 257% below league average.
4.Pedro Martinez 1999 - 221% below league average.
5.Lefty Grove 1931 - 213% below league average.
6.Sandy Koufax 1966 - 212% below league average.
7.Randy Johnson 1997 - 204% below league average.
8.Roger Clemens 1990 - 203% below league average
9.Tom Seaver 1971 - 197% below league average.
10.Steve Carlton 1972 - 176% below league average.
Let's have your feed back. Is my method sound ? what would you do differently ?