Good topic!
I can't speak to other teams , but since I've been working on my White Sox T206 master set for the past 12+ years, I've found that there are several tough Sweet Caporal Factory 25 front/back combos.
For instance, it took me what seemed like FOREVER to find this George Davis combo:
To reference the PSA pop report...there are only 5 SC350/F25 of this card graded, compared to 7 Old Mills (and its one of the tougher OMs!!), and 9 Brown Hindus.
It's not only 350/25s and 460/25s that can be tough - it also took me a very long time to track down this SC150 F25 combo for Fred Parent:
