rats, I disagree re 1984 Donruss. The product was much better on one critical front: design. I am no fan of the post-1980 issues by any means but there are some beautiful cards in that set. The graphics were elegant and well integrated into the design and there were some really dandy photos, especially action shots. It was really a quantum leap above the prior Donruss offerings and the Fleer and Topps cards, which tended to have blocky design elements that broke up the card rather than bringing it together.
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Now, as for the OP question about significant cards 1945-1980, at this point, I'd say these are the big dogs:
1948/49 Leaf Robinson, Paige
1949 Bowman Robinson
1951 Mays
1952 Mays
1952 Topps Mathews
1954 Topps Aaron
1954 Bowman Williams
1955 Topps Clemente, Koufax
Lots of other great cards that are maybe a rung below these.
I base my assessment on how lower end raw versions are doing. I've been watching most of these cards over the last several years as I downgraded my collection and adding these cards at a reasonable price has been challenging. I had hoped to snag a vg-ish 1954 Aaron with nice eye appeal and have watched prices on them rise by about 50%; I finally pulled the trigger on a very appealing PSA 3 yesterday at a price point about 25% more than I had hoped, but I had some good card sales last week and had the extra scratch to do it.