If by iconic you mean historic or classic, that usually depends on the context. Action Comics #1 and Detective Comics #27 are iconic because they are the first appearances of Superman and Batman, respectively. The "firsts" are usually very iconic/classic (first appearance, first issue, etc).
But if you mean it from a more artistic standpoint, that's more subjective. I consider Justice League #1 (1987) and Green Lantern #47 (1994) to be iconic for different reasons.