I've been working on my run for about 4 years, and limiting myself to books in the 6.0 to 9.0 range. Within that grade range, I have yet to see a 2 or 5 for sale or auction. Likewise with 14 and 15. There is actually only one graded 14 (a 6.0) and no 15s in my grade range. If I was picking the six that have been hardest for me, I would likely go with 15,14,5,2,17,18. Everything in the teens are tough though. And the early 20s aren't that easy either. They get even harder when you factor in trying to get ones with nice colors.
Also, the competition for these books seems to be growing and growing! I was sure getting them a lot cheaper a couple years ago.