I'm not asking about the best '70s song here, though perhaps some think there's perfect overlap. I'm talking about the most '70s song. The one that sounds like a perfect time capsule of the 1970s. Like, if a member of Gen Z walked up to you and asked you to explain the '70s, you'd play this song and feel like you'd answered the question.
My answer is Head East's "Never Been Any Reason".
The song manages to both jam and boogie while referencing a dysfunctional relationship and more than casual drug use. It wants to party, but it's also a little fretful of perhaps not making it out of the decade. You can hear the long hair and the bellbottoms. Plus, major keytar rif***e (it's like jamming in space, whoa). Everything about the song is gratuitous, like the decade that spawned it.
Yet the '70s were fertile ground for distillations of place and time. So have at it.