The Garvey tips that came on raisin bran aren't exactly easy. Not sure they'd be on the list as the toughest.
http://postcerealbaseballcardmuseum....rvey-tips.html
The 1979 Red Sox set that was sold outside the park is pretty tough too. I don't know if the standard catalog ever ended up listing the "high numbers" A few cards added on to the set with the late season call ups. Sold as sets in a paper wrapper.
http://www.ebay.com/itm/Mike-Torrez-...AAAOxyThVTYuZL
76 Hostess yellow and green. Different colors, some reversed negatives, numbers over 150, some screened entirely differently...altogether weird. I found one batch when I was in highschool, and none since. Swapped about half of them for the card I'd traded for the whole bunch a 74 McCovey Washington. DOH!
And cheating a bit.......1978 Bazooka. I really need to scan that. Not officially an issued card, but they reused some cardstock with 78 Topps backs printed on it to make some bazooka boxes. I know of 2, maybe someone else saved one?
Steve B