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JoshThose site are indeed good to check out. I set up the Yahoo one.
Anyways, to answer your question, all sets are incredibly difficult to build, and that is not even taking condition into account. The majority of the cards are PR-FR-G because of back damage, glue or tape residue, staining, and wear in general. VG and VG-EX cards are often very nice because they generally lack major damage. EX and EX/MT cards are extremely nice and represent an extremely small percentage of the cards that are found today. And for the rare ones that are NM or better, they are truly exceptional cards.
That said, it is difficult to rank the sets in terms of how tough they are to build because each set has its own nuance that makes it incredibly hard to complete.
The 1966 and 1964 cards are seen in more of an abundance than the other years, but those two particular years are more likely to have major back damage than any other years.
The 1959 cards are seldom seen and are very difficult to find, same with 1960. The 1962 cards have their own issues as well due to a packaging flaw, so they are often heavily damaged, however, there are no particular cards in the 1962 set that were issued in any lower quantities than any other card in the set.
The 1967 MLB portion of the set is only 150 cards, but it is extremely difficult to find certain cards from that set. The 1968 set has a couple cards that are very elusive, while the rest are not that difficult.
If you are not a member of the Yahoo group, you should definitely join it. I have put up a ton of info on there, and there are also lots of great pictures.
-Josh