for me, it has been Zeenuts.
What a fascinating issue. I almost think that there is less known about some of its nuances than other Super Sets like the N172 Old Judges
I'm particularly keen on the tabbed variety, as I specifically wanted to choose something to keep my interest for a decade or more. But the more examples I obtain, the more interested I am in learning more about the entire series:
Here's a few on my mind at the moment:
1924, with its full photographic images:
The dentist, "Doc" Farrell:
The one-legged catcher, Gilly Campbell: (just bad cropping!)
The gorgeousness of the 1915's -- simple aesthetic beauty!
The not so famous son of HOF'er Ed Walsh:
And, of course, one of the most famous of PCL HOF'ers:
And since we're talking about HOF'ers, here's the predecessor to Bo Jackson, a Pro Football HOF'er!