There are many, many songs that fit the bill here.
In terms of my fave's career, I don't know if they're "ICONIC!!1!" amongst the "GP" or whatever, but Burning Up, Oh Father, Human Nature and What It Feels Like For a Girl have all become quintessential Madonna songs. And a special mention must go to Into The Groove, which despite not actually being released as a single in the US has become a classic.
