My third choice after the Creighton and Kelly would be the Just So Young. Like Four Base Hits, Just Sos are incredibly rare; for a type collector getting a common is very difficult. What makes the Young so special is that it is the only 19th century card (excluding studio cabinets) of arguably the greatest pitcher of all time, and shows him young and trim near the outset of his career. Another ten years will pass before his next card.
I agree with Jay; this is a fun debate.
