I've been at this for (too) many years and I have found the hardest to be:
Negro Leaguers, Pioneers/early stars and certain non-players. Some do not even have cards issued from during their careers.
Here is a list of
some of the most difficult from each category.
Negro Leaguers generally require international Cuban and sometimes Puerto Rican or Mexican issue.
There are people who know much much more than I do about this and I hope they will share there wisdom. Here is my gloss on the harder ones that are out there:
Pete Hill (only a few exist), Andy Cooper, Biz Mackey (basically one issue), Oscar Charleston (not nearly as rare but very very expensive), Jose Mendez, Christobal Torriente and "Pop" Lloyd
These are generally thousands of dollars in any condition and they are not likely to be found in "high grade."
Significantly, many of the great Negro League players do not have cards issued in any "sets" during their careers and these include Josh Gibson. There was a card issued soon after he died in 1950-51 by Toleteros (sp?) that has been celebrated.
Pioneers/ Pre-1887 greats - A
.G. Spaulding, George Wright, and Candy Cummings are all very hard to find and the best bets are CDVs and "team cards." These are also thousands of dollars. There is a one-of-a-kind G&B card from the later 1880's of Spalding from his years as an Exec./Owner.
After 1887 individuals cards exist for just about everyone who
played in the majors. The earlier players are harder to find -- such as
Bid McPhee and Deacon White.
FYI :
Jesse Burkett generally requires a bit of a cheat to get a career card -- the T204 was issued after he finished in the majors and for whatever reason he is not in the Mayos or the Fan Craze sets. There are some very rare premiums of him and a one-of-a-kind "Just So" card.
Of the last group -- umpires execs etc.. Some of the most difficult that were issued during their careers are:
Barney Dreyfuss long-time owner of the Pirates who shows up in a Tip Top Bread set from 1910 commemorating his team's success the year before.
AL Founder Ban Johnson - He's in the Fan Craze set but that's about it.
Umpire Billy Evans - only in the 1922 Exhibit set;
Umpire Bill McGowan -- in a card set issued by his umpire school in the 1949-50 and I think (not sure) something called the Safe-T set.
Manager Joe McCarthy is not particularly easy to locate. For whatever reason he was not given his own card in any of the major sets during his illustrious career. Strictly speaking the only card (as opposed to stamp or button or coin) of McCarthy card is in the 1936 World Wide Gum Card set.
Umpire Hank O'Day has a card as a player in the Old Judge set (before he became an umpire)... He was a lousy player.
Manager Frank Selee has a one-of-a-kind card from a game set issued in 1904:
http://www.net54baseball.com/showthread.php?t=75560...
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Also, I learned that there are cards of recent HOF inductee
John Schuerholz from his tenure as the Royals GM (team issues "photo-cards" from the early 1980's) before he went on to greater fame as the GM of the Braves.. If any one has one I could use one.