It might be easy to find the best one year performance. Just check a reference like the Baseball Encyclopedia and see who had the most wins, lowest ERA, lowest ERA relative to the league average, etc. But what about over a stretch of time, say five years? To try to answer this, I found the best performances over a five year period in RSAA using Lee Sinins' Complete Baseball Encyclopedia. Here is the definition: "RSAA--Runs saved against average. It's the amount of runs that a pitcher saved vs. what an average pitcher would have allowed." It is also park adjusted, so that pitchers who pitch in good hitter's parks get an adjustment upward in their RSAA, and vice-versa. A below average pitcher will have a negative RSAA. I only looked at the years from 1920-2005 so as not to include the deadball era. That should probably be a different article.
The table below shows the highest cumulative RSAAs over a five year period. So the time frames for each pitcher are consecutive years. This is not just the best five seasons fore each pitcher added up.
The "ending year" column tells us the last year of the five-year period. For example, the 1932 for Lefty Grove means the period was 1928-1932. I meant to have only the top 25, but there is a three-way tie for 24th. Newhouser's two entries both include at least one season from the World War II years (1943-45 when many of the best players were in the military). But his 1949 streak only includes one of those seasons, 1945. So maybe he belongs.
Of course, for some of these cases, seasons overlap. Obviously Grove 1932 and Grove 1933 shares some seasons. The table below shows only distinct and separate 5-year periods. So some of the cases above are taken out, like Grove 1933 since it overlaps with 1932, which was better.
Some pitchers actually had two separate and distinct 5-year periods among the all-time best. Grove has his 1928-1932 period as well as his 1935-1939 period and they are both in the top 3! And until Pedro Martinez came along, Grove had the two best all to himself. I left both of Newhouser's seasons in so you could see that he did well when including only one World War II season and still see his best streak.
But in some years it takes more runs to win a game because the league average is higher. To adjust for this and find how many extra wins each pitcher had above the average, I used the formula which says it takes 10 times the square root of the number of runs scored per inning by both teams (found in Total Baseball, 5e). If each team scores .5 runs per inning, the total is one. The square root is 1 and 10 times that is 10, so it would take 10 additional runs over the course of a season to win one more game. Then I calculated how many wins each pitcher added per season, then found the best 5 consecutive year periods again. For example, Pedro Martinez had 77 RSAA in 2000 when it took 10.89 more runs to win a game. Since 77/10.89 = 7.07 he got that many "extra wins" or wins above average for that season. Then, of course, I found all the 5-year totals and ranked them from highest to lowest.
From 1928-32, Lefty Grove won 28.88 more games than average, based on his RSAA and the formula for runs per win explained above. Again, there are many overlapping seasons. The next table takes them out, like I did in the case for RSAA.
Newhouser's two seasons were left in for the same reasons mentioned above. Again, Grove, Randy Johnson and Roger Clemens all had two separate and distinct five-year periods.
One factor that RSAA does not take into account is fielding. A pitcher with better fielders will allow fewer runs, so some of the runs that these pitchers saved, along with the games they won, are attributable to their fielders. Perhaps some of these guys should not rank so high while some others should be higher and did not make the top 25 or 30 if we could separate out the contribution of the fielders. But many of the pitchers here also rank very high in categories like strikeout-to-walk ratio relative to the league average and HRs allowed relative to the league average. The table below gives us an idea of how high some of these pitchers ranked in these stats. The ranking was based on all pitchers with 2000 or more career innings pitched from 1920-2005
Grove had a career strikeout-to-walk ratio of 1.91 while the league average was .9. His ratio was 2.11 times the league average and that was 4th best. Grove allowed 162 HRs in his career while the average pitcher allowed 232. That is a ratio of .698 (162/232). That is the 15th lowest ratio. The difference is 70, and that was 19th best. The X means that a guy was not in the top 50 in a category. Generally, these pitchers showed that they could hold down opponents' runs no matter who the fielders behind them were since they rank so high in strikeout-to-walk ratio relative to the league average and HRs allowed relative to the league average. That indicates that they were quality pitchers (or run preventers) independent of their fielders.
Technical notes: I first compiled a list of all pitching seasons with at least 7 RSAA. In case one of the top 5-year periods got left out, I looked at the career records of all pitchers who had at least one season with 40+ RSAA. Since I was looking for consecutive 5-year seasons, if a guy had, say, a sequence of seasons with RSAA totals of 70, 70, 6, 70, 70, his 5-year total would be 276 but he would have initially gotten left out. The only pitcher who ranked highly that I found by this secondary check was Lefty Gomez who had a sequence of 57, 22, 3, 69, 41. Bob Feller would have had a 5-year total of 194 over the years 1939-41, 1946-47. The missing years were for World War II. He did pitch in 1945 after his military service ended but he only pitched 72 innings that year. Randy Johnson in 1998 split some time between the Mariners and the Astros. When I compiled the list of pitchers I did each league separately. Johnson had 4 RSAA with the Mariners and that could be added to his total for the 1998-2002 period. It would go up to 270 but it would not change his rank.There were no other top pitchers on the lists here who split time one in a single season between the two leagues.