Getting more into signed items/game used recently, so my favorite/best isn't a card but rather a team signed ball.
1955 Brooklyn Dodgers
I love it not just because it is a 55 Dodger ball, but also because I can date it to the first month of the season. Koufax started the year on the DL and Tom Lasorda was brought up to fill his spot. Lasorda pitched fewer than 5 innings over the first month before Koufax came back and Lasorda was sent packing to Montreal. So, the presence of Lasorda dates this ball to that period, and the presence of Koufax means that he signed the ball before throwing a single pitch in a regular season pro game. I just love that! Plus, the only clubhouse is Duke Snider...Brooklyn was notorious in the late 40's through 1957 for having a clubhouse guy who would sign for many of the players...so a ball with just one clubhouse is pretty good!
Tom C