My 3 favorite pieces are all autographed baseballs. Mantle and Mays the most valuable, but by far my favorite piece is a John Franco signed baseball.
I don't know that many will remember when he got suspended for a few games several years ago for bumping an ump, but he did. He was not allowed in the dugout, so he sat behind the plate with 2 scouts. I was at that game with my father, and somehow, managed to be sitting right next to him.
He only sat there for the first four innings, but he signed a ball for me and we even had some cursory chat about the game. For a life long Mets fan, it was one hell of a moment.