I'm surprised that so far nobody has listed Ganley or Tenney. Both, especially Ganley, were a little difficult for me.
SLers can be troublesome at times, especially if you're looking for good examples of the Old Mill back (Piedmonts seem to be everywhere). And of course the Hindu versions, especially in good condition, are the most difficult of the SLers. I didn't see the backs being a considering factor in the aforementioned list.
As someone who's nearly completed the monster, I must say most of the cards listed are easily found on eBay or Net54, sans the big 4. Perhaps it was more difficult putting together the monster pre-Internet, but I've found it relatively easy. Ridiculous BIN prices, finding extra cash and dealing with a nagging wife has been my biggest hurdles.
