The Greatest Riff of All

Was just reading a review of the latest re-issue of Jethro Tull's Aqualung, which posited that the opening riff of the title track was the most arresting and best rock riff of the seventies , out riffing even Deep Purple's "Smoke on the Water".

Once you start thinking of this lots of thinks start crawling out of the woodwork , such as "Layla" by Derek and the Dominoes , "Black Dog" by Led Zeppelin  , any number of Black Sabbath and Hawkwind songs.