Many fans know that Norm Cash batted .361 in 1961. He also had 41 HRs and 132 RBIs. Never batted .300 again (his last year was 1974) nor did he ever reach 40 HRs or 100 RBIs. Perhaps this is the most atypically good season ever. He clearly performed well above what ended up as being his career norms (was it the corked bat mentioned in the ESPN almanac? I recall that physicist Robert Adair said a corked would not really help).
Anyway, to study this, I looked at all players with 10+ seasons with 400+ PAs through 2005 (there were 504 players). I found the simple mean of their yearly offensive winning percentage or OWP (a Bill James stat that says what a team's winning percentage would be if all 9 batters were identical and you gave up an average number of runs). Since I used data from the Lee Sinins complete baseball encyclopedia, OWP is also park adjusted. Then I subtracted that mean from their best year. The following table shows the top 25 in terms of best minus average OWP. Cash's 1961 season was 25th. Another table follows that only looks at seasons since 1920.