Most Important Baseball Card!

[G1911]

The first baseball card isn't much important, as by 1869 the CDV era was in full swing and cards were very popular on both sides of the Atlantic. The focus of cards was just not on sports yet and it wasn't significant in its time.

The T206 Wagner was significant decades after its issue, but not much at the time. It didn't drive the T card explosion at all.

The 1952 Topps Mantle is the most important single card in a significant set, but its really Bowman that launched and pioneered the post-war card boom and while 1952 Topps values today might be a bit lower today if it did not exist, it didn't really have an impact on Topps winning the war and becoming the sole mainstream maker for decades.

Most of the big moments of card production history aren't because of a single card, and were often launched by non-baseball cards. Cards as cigarette inserts begin with non-sports, the picture card hobby begins with non-sport cards and explodes in popularity with non-sports. The post-war gum card popularity isn't the result of any particular card. Boring, but I cannot think of any baseball card that if it did not ever get made would have significantly changed card history. Hard to assign one as "most important". I'd vote Wagner for most iconic, Mantle #2.