The first card is easily a no-brainer.
1952 Topps #311 Mickey Mantle
This became the signature card for Topps in the baseball card market. That year Topps put out their inagural set and, unlike Bowman, used color photography. This card quickly became popular to baseball fans and collectors alike. How much impact did it for Topps? From 1997 to 1999 and 2001-2005 Topps did not issue card no. 7 in honor of Mantle. In 2006 they issued card no. 7 with Mantle on it where it continues on.
If you want point the beginning rise of Topps look no further than this card.