Since I don't have any expensive ones.... all of them?
I suppose the favorite for a while is this ball I got for 50 cents at a flea market from a dealer who told me it was "little league or something" And tried to upsell to ones signed with felt pens so wide they were almost paint brushes.
I figured even a written on minor league ball had to be worth 50 cents. (And this was only like 3-4 years ago)
I like it because he was between Seattle PCL and NY- probably just drafted by the Giants. And whoever got it used a ball from a league he wouldn't play in until 1957.