In 1961, the American League expanded to 10 teams, followed by the National League expansion to 10 teams in 1962. The league would remain at 20 teams until 1969.
There is simply no denying just how dominant Sandy Koufax was, if even for a short time. His performance from 1963-1966 is the greatest stretch for a pitcher ever, as he went 97-27 with a 1.86 ERA (172 ERA+) over that span.
Good as that was, it still didn't knock Hammerin' Hank Aaron out of the top spot in a decade that saw the primes of four 500-homer run hitters in Aaron, Frank Robinson, Willie Mays and Harmon Killebrew.
Dominant as Koufax was, Bob Gibson's 1968 season goes down as perhaps the greatest of all time, as he went 22-9 with a 1.12 ERA, prompting the mounds to be lowered. Juan Marichal and Don Drysdale were phenomenal in their own right, though no doubt overshadowed a bit by Koufax and Gibson.