This is the hardest one of the decades for me. Do you go with the big rock bands of the 1970's (in which case I'd have to pick some Zeppelin, maybe "Stairway" or "Heartbreaker/Living Loving Maid" -- the later simply for it's opening guitar riff, which just screams "ROCK!" and sets the stage for heavy metal) or the disco era? For disco, you almost have to go with the Bee Gees. There's also the funk/soul element. Stevie Wonder's "Superstitious" is a strong candidtate, but probably gets beat by Marvin Gaye -- "What's Going On?" or "Mercy, Mercy Me" Marvin was genius.
However, one musical (sub)genre emerged from the 1970's that had a very significant musical and social impact and a huge personal impact on me as well -- punk. So, in the end, I have to go with:
"London Calling" by The Clash -- 1979.