Just catching up on this thread. I mostly concur with Kelly and Rick.
Interesting, regarding the difficult books, it is amazing how (relatively) accessible most all of the Planet issues have become, no doubt due to increased collector interest in Planet Comics and the rising prices motivating owners to sell their copies. When I got into the Planet game 10-ish years ago, I was lucky to find 3-4 books a year, and rarely the rivets. It was only in the last few years when I was able to fill most of my collection. It's amazing how so many boardies have been able to build up their collections in such a short time.
So here's my own perspective, based on my 10 years of slow-poke collecting, and more recent trends:
1-9: issues 6, 8 (with 2 and 5 close behind)
10-19: issues 10, 15 (with 14 and 17 close behind)
20-29: issues 20, 24
30-39: issues 32, 34
40-49: issues 40, 45 (all generally not so HTF)
50-59: issues 51, 56 (all generally not so HTF)
60-73: issues 63, 65 (with 66 and 70 close behind)