People only really remember the most recent ones, but I think Dolley Madison is pretty iconic for saving the White House painting. Lady Bird Johnson is also pretty famous in Texas, still.
But I agree, Hillary Clinton will obviously go down as the most iconic since she is senator/Secretary of State/probably President.
Michelle Obama may not compare to the others as of now b/c she is the current FLOTUS
but i think she will be iconic as we look back at history years from now...she's definitely had more of a presence & impact than her predecessor Laura Bush.