all the issues in the 60's are "tough", followed by the 50's, and lastly the 70's... in fact, there are really no "tough" issues in the 70's, as long as you are not condition sensitive...I see every issue come up for sale multiple times in a year...
now, I limited myself to only completely unrestored copies ...
for me, it was issue 67...took me 5 years to find one (I found one with replaced staples, so it was a green label)...didn't see a single unrestored copy in the market... then, as fate would have it, after I paid OUT THE NOSE for a copy (in an auction, against someone else that obviously hadn't seen one in forever), I have since seen 4 copies available in the last year...
61 has traditionally been tough (maybe 2nd toughest)...