As far as rarity goes, here's my list of the harder things to find in my experience:
The Back Issues is a bit tough, no idea how many were printed, but they don't seem to be very plentiful.
The second printing of 3-D Zone #16 is a tough one as well. No number or Canadian price on this one. (Free in Canada?!)
Seduction of the Innocent Non-3D #1 - I believe there were only 100 copies printed of this one, I've been searching high and low and have yet to come across one. As a completionist I would give a tooth, or some other body part for a copy.
Men of Mystery #202 - Fanzine cover for a friend of Dave's wedding, with a personal note from Dave on the cover. This cover was also printed in Selected Sketches & Studies #3 and is of Captain America. There were only 50 printed, and 49 out there, since I have #2 of the 50