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martindl"Important" is such a subjective term. In fine arts, it is used to define an artists' masterpiece, first of a particular genre, a piece that defines that particular artist, an artist as catalyst of a particular movement, etc. When applied to cards, it could mean many of the same things, as i think many of the individual cards listed above do.
For me, I very much like Jay's list of sets and where they fall in the order of things. I'd probably add either the 1959 Fleer Ted Williams set or the Fleer 1963 set, because from an importance perspective, both were attempts to break Topps' monopoly.