I’m gonna back my 10-year-old post and once again nominate, “Chevy Van” as the most 70’s song. It has staying power, because it IS the most 70’s song.
Didn’t even realize it was a 10 year old resurrected thread until you mentioned it. 
Wow, ok old thread about old music man, TURN IT UP!
Rock Me Again and Again and Again and Again and Again and Again by Lyn Collins 1974.
At the very least you can’t get any more 70s than that song title.
Any song performed by Gordon Lightfoot.
I’d agree. That song just reeks of the 70s: anonymous pre-AIDS no-worries sex, the “shaggin’ wagon” 1970s van culture, hitchhiking still being an accepted way to get around, and a singer-songwriter with an acoustic guitar. Neither Gordon Lightfoot nor John Denver could define the 1970s as well as Sammy Johns did with “Chevy Van.”
When I saw thread title, I’m pretty sure my brain tried to come up with this song, but somehow “Dust in the Wind” took hold instead. I was getting ready to post it, but thought I’d read the others first. Then I saw this, played each, and Carry on Wayward Son is clearly the winner. 
Lou Rawls - You’ll never find
I love this song but its about as '70’s as you can get
Andy Kim - Rock me gently
By that criteria, I would nominate “After the Goldrush” by Prelude:
This was one of my favorite songs of the era, and after it disappeared from the radio, I’d remember snatches of it but never enough to find it again. I finally did a project where I attempted to listen to every top 100 song starting in 1970, to find this and other songs I’d forgotten.
When I worked on the radio, we announced this with “Special dedication, goin’ out from OJ to Nicole.” 
I’d rather post 70’s songs that were actually good. As others said, I don’t think the 70’s had a ‘sound’, as the music was just all over the place. It was a very experimental time, and some of the best music of the 70’s is pretty timeless.
Here are some that really bring back memories of my 70’s:
Blinded by the Light - Manfred Mann’s Earth Band
Hotel California - Eagles
Werewolves of London - Warren Zevon
That Smell - Lynyrd Skynyrd
Flirtin’ With Disaster - Molly Hatchet
Money - Pink Floyd
Barracuda - Heart
Hello in There - John Prine
Rocky Mountain Way - Joe Walsh
Tush - ZZ Top
Sweet City Woman - Stampeders
Sunny Days - Lighthouse
The 70’s is still my favorite era of music. There was lots of absolute dreck on AM radio, but then there always was. FM rock in the 70’s was awesome.
“Disco Inferno” — The Trammps (“Special dedication, goin’ out to all you Branch Davidians out there!” 
)
“No, Honestly” — Lynsey De Paul
“If I Can’t Have You (I Don’t Want Nobody, Baby)” — Yvonne Elliman
“Yes, Sir, I Can Boogie” — Baccara
“Parlez-Vous Francais?” — Baccara
“Sorry, I’m a Lady” — Baccara
“Mississippi” — Pussycat
“Don’t Go Breakin’ My Heart” — Elton John and Kiki Dee
“Back in the USSR” — The Beatles (Yes, I know it’s older, but it was freakin’ everywhere in the UK in the summer of 1976!)
“You’re So Vain” — Carly Simon
So was “Got to Get You Into My Life” (66 version) if I remember correctly—so much so that they rereleased the single in the mid-70’s, when I bought it.
Does the song have to be from the '70s?
I was going to say Afternoon Delight, because it hit on a couple of themes common to 70s music and the 70s zeitgeist:
One-hit wonders
White people singing coyly about having sex, especially maybe out of wedlock sex!
However, as much as I dislike the song, I think Chevy Van may be a better choice. In addition to:
One-hit wonders
White people singing coyly about having sex, especially maybe out of wedlock sex!
It includes the themes of
Automobiles
Hitchhiking
Country influenced sound.
Meta reference to rock&roll
It’s almost like the perfect light rock song for the 70s.
I can say that only two songs really are associated with specific memories in the 1970s. One is “Macho Man” by the Village People, mostly because I recall it being on the PA in the five-and-dime when I was a little kid at one point.
The other is “Dust in the Wind”, and I remember that because it was on the car radio when I was probably 4 or 5, and I remember thinking it was the most horrible, sad and depressing song I’d ever heard. I actually resolutely avoided the song for the better part of the next two decades, as a result of that listening experience.
“Have You Never Been Mellow?” — Olivia Newton-John
'Cause everyone was like, you know, so mellow back in the '70s. 
My problem is that I associate “Chevy Van” with Chevrolet commercials from subsequent decades.
I really want to say it is “Aquarious: Let the Sun Shine In” but that is 1969.
Feels very emblematic of the 70’s to me though I was born in '67 so my memory doesn’t really start till 1970(ish).
Help Me by Joni Mitchell.
“Please, Mister, Please (Don’t Play B-17!)” — Olivia Newton-John.
Fer cryin’ into yer beer. 