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BenAll of those sets/cards are extremely tough. On a per card basis, I would say that the t206 uzit/drum, D359's, red crosses, and maybe the plows candy's are the "easiest". I don't think that e94 overprints or e98 old puts count as a legitimate SET, so it would be somewhat misleading to rank them on the same grounds as the others.
After those, it's pretty much a wash like pete says. You can pretty much file the rest in the "insanely difficult" category and leave it at that.