Best ERA+ is a season ever (shown below). Not a perfect indicator, but helps normalize the data across eras (by comparing ERA to others in the same season).
Keefe's 1880 season he only pitched 105 innings, but BP includes pitchers that pitched more innings than the team played games, and the Troy Trojans only played 83 games that season which is why he is on the list.
Pedro's 2000 season tops the list by a decent margin. Leonard's 1914 he went 19-5 with a .96 ERA.
No one in recent memory will have the counting stats that the players in the late 1800's/early 1900's had, as it is a different game today so there's definitely arguments either way. For me, it's Pedro's 2000 season.
Rank Player (age that year) Adjusted ERA+ Year Throws
1. Tim Keefe+ (23) 293 1880 R
2. Pedro Martinez+ (28) 291 2000 R
3. Dutch Leonard (22) 279 1914 L
4. Greg Maddux+ (28) 271 1994 R
5. Greg Maddux+ (29) 260 1995 R
6. Walter Johnson+ (25) 259 1913 R
7. Bob Gibson+ (32) 258 1968 R
8. Mordecai Brown+ (29) 253 1906 R
9. Walter Johnson+ (24) 243 1912 R
Pedro Martinez+ (27) 243 199
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