For me, the tough cards are the 1908-09 Dietsche postcards. They are not as tough as some of the other cards mentioned in this thread, but they are not common either!
A group of these cards just came up in the last Heritage auction, but I didn't have the funds to purchase them at that exact moment since I was negotiating to purchase some Worch cards, which can be fairly tough too.
If anyone has any, I do have some cash and trade bait available
