Different cards have different meanings to me, but there are a couple that I favor over others.
1985 Topps Kirby Puckett rookie - the first Puckett card I ever owned.
2008 Topps Stadium Club Joba Chamberlain (1st Day issue) - the first Joba card I acquired. It is a favorite because I was at the game with my father (a lifelong Yanks fan and his first trip back to "old" Yankee stadium since he was a kid), and remember exactly how the play developed that led to picture on the card.
1988 Topps Kirby Puckett - for Xmas this year, my wife went to a local shop that has baseball cards (definitely not a card shop) and picked up a Puckett card to give to me as an Xmas present from my than 2 year old. Good thing I didn't need it for my collection, as it never made it past my sons hands. He added it to his collection that he carries around in a lunch box and dumps out on the floor just about every night.