From my experiences, the toughest by far is the 1959 Creme backs followed by the 1959 Gray backs, 1968, and the 1960. After that, I have found them relatively easy and available. Here are a few of my favorites (the 59 is a gray back):
Not a bad scan... It actually looks like that.
The Venezuelan is on the right. The other is a poorly trimmed U.S. issue I have to upgrade at some point...
