I'll give you two iconic cards in my collecting history. One is the 1967 Topps Tom Seaver RC. Man that is a tough one to find being a high #. I've always held that card in much higher status than the more heralded Ryan RC.
Another iconic card, also from the 1967 Topps baseball set is the Brooks Robinson. In the very earliest days of price guides that '67 Brooks was almost always on the covers price guides or just books about baseball cads. THis is when there were no monthly guides and certianly no OPGs but just a handful of annuals. Trying to find the earliest of price guides that did not have this very hard to find Brooks Robinson card was rare.
