Since the 1970's, Bohemian Rhapsody is now the best seller which originated from that decade due to re-releases, etc but Mull Of Kintyre was the best seller at the end of 1979.
Candle in the Wind was first released in the 1970s as part of a double A side, and then sold many millions in the 90s but I don't think that counts as it was a re-recording