What are some of the toughest mainstream vintage cards to find?

[Leon]

John Ramirez had told me several years ago that he had a T231, as we were having breakfast at the National. Approximately 8 mos to a yr ago he showed it on Bob L.'s blog. AWA Caramels were only known as a single example until board member Dan M. showed 2 more examples from a private collection, a few months or so ago. The recent T207 Red Cross's that were found approx. doubled their known population of 5. The only problem with collecting the rarest of the rare is that finds will happen. I still love it though and wouldn't change a thing.
As far as mainstream pre-wwII that are difficult to obtain.....the 14' Matty CJ might be there....they are around but not too often....maybe 1-2 times a year.....same with the T210 Joe Jackson.....and one that is also rarely seen is one I was happy to pick up recently but don't have yet, the T210 Kelly Mascot....good question.

edited to add, Griiffin's post right above really hit the nail on the head