This has been discussed before, but always fun to re-visit. As collectors, our opinions/perceptions of book difficulty are based on many factors .... amount of time collecting a run, amount of cons/comic shows attended, amount of $$ in pocket at particular times, relationships with dealers, time spent delving into the various auction houses offerings, and a little thing called luck.
I have collected Planets since 2006 and I mostly chase 8.0 or higher, but make note of any Planet I see in private hands or for sale / auction. To answer your question about which books are super tough, I broke the run down into 7 groups 1-9, 10-19, etc and have listed below the two most difficult books in that group of books that I have found in my time collected Planets. Now for clarification, this is most difficult not most expensive. Sometimes these two do not go hand in hand.
You may agree, you may disagree, but it should be fun “discussing” if anyone wants to share any opinions.
1-9 Issues 6,8
10-19 Obviusly all are difficult and yes 13,14,and 15 are all equally tough so between the rest of the issues in this group it would be 10,19
20-29 Issues 20,24
30-39 Issues 30,38
40-49 Issues 40,47
50-59 Issues 51,54
60-73 Issues 61,72