From 1903 to 2011, from 1st season to 6th season, (requiring onbase_plus_slugging_plus >=150 and At least 3000 plate appearances)
Twenty batters found, most in the Hall of Fame.
I know this old guy who really likes Joe DiMaggio. He often says that DiMaggio had the best first six seasons of all time. Actually, DiMaggio is tenth, still pretty good.
Using the first six seasons is obviously arbitrary. DiMaggio fell off in season seven, which is why his fans exclude it. Also, other players may have played partial seasons, especially the first. Mickey Mantle is saddled with his weak rookie season (one month in minors) but still finished sixth. The Mick would have been higher if this started with his second season, when he led AL in OPS at age 20. Mantle got OPS+ 165; career 172. Willie Mays is similarly impacted because of his military service. And, of course, Babe Ruth did not even make the list because he was busy pitching before setting the season home record in 1919, his sixth season, .
DiMaggio's old nemesis came in first: Ted Williams for seasons 1939-1942, 1946-1947; military service did not slow him down. Williams was aided by the Red Sox shortening a long stretch of right center field in Fenway Park in Williams second season.
What if we look at the first seven seasons of batter's careers? Seven seasons fits DiMaggio's career perfectly (1936-1942) before he, too, entered the military 1943-1945. Why don't DiMaggio's fans suggest that? Aside from being tech and knowledge phobic, they have no criteria for their claim. I suggest OPS+. What do they suggest? Testimonials from well known non-baseball people. Maybe batting average. The data for the first seven seasons produces 29 batters with OPS+ >= 150.
Williams (OPS+ 195) is still number one. Mantle (OPS+ 173) slides up to number 5. DiMaggio (OPS+ 159) drops from 10 to 15.
First six seasons: 25 batters since 1903 had BA at least .325 based on 3,000 PA. See data. DiMaggio was 8th with .345.
Chuck Klein .359.
Joe DiMaggio definitely did NOT have the best first six seasons of all time.