Sticking with " regular " issues, the 71 Topps is expensive, but not difficult, due to the black borders. The 72 set is difficult due to centering issues 9 if that's a criteria for you ), and the 77 set is difficult due to thinner card stock, some centering issues, and the cards are prone to chipping.
The test sets mentioned previously are darn near impossible to find at times, and when found are very expensive. So set completion for those items likely require some deep pockets. Unless they were purchased " back in the day ".
