One of the delightful treats of collecting Topps cards was how they distributed the players to the checklist numbers. Good players generally had a number than ended in “5”, All-Stars ended in “0”, and the very best players were assigned multiples of “50”. This was never announced, it just happened and kids took it on faith. In fact, if you learned the game as I did — from the cards — Topps assignments helped you figure out who the best players were. Willie Mays had a multiple of 50 every years between 1959 and 1965 (before I came on board).
In 1966 he got #1, one of the few times Topps used that number to anoint a superstar. In 1962 they gave the first card to Roger Maris, fresh off his 61 home run season, but in the intervening four years Topps had put its leaders cards at the front of the set. But in 1966, they gave it Mays who had just had one of his greatest seasons.
Source: Mark Armour, SABR’s Baseball Cards Committee