The flutes and gnomes and shag-carpet make-out sessions inspired by "Stairway to Heaven" (the most-played, most-requested song on American radio, despite never being released as a single) made Zeppelin's fourth, officially untitled record the band's last wholly formed masterpiece and a cultural bellwether of the '70s. All wheat and no chaff, the record's influence on heavy metal and rock can't be overstated, from the weirdly enchanting medievalism of "Battle of Evermore" and Joni Mitchell-dedicated "Going to California" to the flawless testament to Bonham's genius, "When the Levee Breaks."