Babe Ruth is an essential addition to the list. This card was issued in 1914, just after the fresh-faced rookie was signed to the Baltimore Orioles, then a minor-league team. The cards were issued in either red-and-white or blue-and-white color schemes with a game schedule printed on the back. Only 10 of these cards are believed to exist. A few months after this card was issued, Ruth was traded to the major-league Boston Red Sox and began building his legend as one of the MLB's greatest homerun hitters.
Sportscard Guaranty graded this card using a scale of 100. While it garnered a relatively low grade of 40 (no high-grade versions are believed to exist), the card garnered 59 bids during a rare auction appearance in 2008. It sold through Robert Edwards Auctions for $517,000 [source: Sports Collectors Daily].