Obviously, Stairway to Heaven is number one. (It came out in 1971)
Other options:
Won’t Get Fooled Again
Band on the Run
Roundabout
The Low Spark of High Heeled Boys
My Sweet Lord
Truckin’
Candle in the Wind (originally released in 1973)
I came back into this thread specifically to mention that.
Guess I’ll have to mention Magic Man and Barracuda instead.
Y.M.C.A.
My Sweet Lord, George Harrison
Bridge Over Troubled Water, Simon & Garfunkel
Thank You (Falettinme Be Mice Self Elf Agin), Sly & the Family Stone
Mr. Bojangles, Nitty Gritty Dirt Band
Put Your Hand in the Hand, The Ocean
Uncle Albert/Admiral Halsey, Paul & Linda McCartney
The Low Spark of High Heeled Boys, Traffic
Your Move, Yes
Free Ride, The Edgar Winter Group
River Man, Nick Drake
You Are the Sunshine of My Life, Stevie Wonder
Superstition, Stevie Wonder
As, Stevie Wonder
I Saw the Light, Todd Rundgren
Born to Run, Bruce Springsteen
Jungleland, Bruce Springsteen
Meeting Across the River, Bruce Springsteen
Takin’ It to the Streets, Doobie Brothers
The Ballroom Blitz, Sweet
Bohemian Rhapsody, Queen
Mockingbird, James Taylor & Carly Simon
The Best of My Love, The Eagles
Midnight Train to Georgia, Gladys Knight & the Pips
Lady Marmalade, LaBelle
Ventura Highway, America
That’s the Way I Like It, K.C. & the Sunshine Band
Rock and Roll All Nite, Kiss
Surrender, Cheap Trick
I Want You to Want Me, Cheap Trick
Low Rider, War
Baba O’Riley, The Who
Won’t Get Fooled Again, The Who
Walk This Way, Aerosmith
Crazy On You, Heart
Blind By the Light, Manfred Mann’s Earth Band
Show Me the Way, Peter Frampton
The Boys Are Back in Town, Thin Lizzy
Turn the Beat Around, Vicky Sue Robinson
Play That Funky Music, Wild Cherry
Dancing Queen, ABBA
Rock Lobster, The B-52’s
Rock ‘N’ Roll Fantasy, Bad Company
Roll With the Changes, REO Speedwagon
Go Your Own Way, Fleetwood Mac
Jive Talkin’, Bee Gees
Fooling Yourself, Styx
One Way or Another, Blondie
Paradise by the Dashboard Light, Meat Loaf
Da Ya Think I’m Sexy, Rod Stewart
Miss You, Rolling Stones
Brown Sugar, Rolling Stones
It’s Only Rock ‘n’ Roll, Rolling Stones
The Time Warp, Rocky Horror Picture Show
Boogie Oogie Oogie, Taste of Honey
I Need a Lover, John Cougar Mellencamp
Message in a Bottle, The Police
Changes, David Bowie
Young Americans, David Bowie
Rebel, David Bowie
Instant Karma!, John Lennon & Yoko Ono
Stairway to Heaven, Led Zeppelin
Brown Sugar and Miss You by the Rolling Stones
And since the OP asks “What songs would 90% of the population be able to relate to and instantly associate with the 70s?” it’s impossible not to mention the Bee Gee’s five #1 hits off the Saturday Night Fever soundtrack (and two more that were written by them). If I had to name just one as an icon I’d go with Stayin’ Alive.
Also one of the great songs of the 80s!
Off the top of my head, these are the first 25 songs that I thought of from the 1970s:
Hot Rod Lincoln - Commander Cody & His Lost Planet Airmen
Miracles - Jefferson Starship
Evil Ways - Santana
Feel Like Making Love - Bad Company
Reeling In the Years - Steely Dan
I’m Not In Love - 10cc
Love to Love You Baby - Donna Summer
You Sexy Thing - Hot Chocolate
Me & Bobby McGee - Janis Joplin
Psycho Killer - Talking Heads
Gloria - Patti Smith
Rhiannon - Fleetwood Mac
Freeway Jam - Jeff Beck
Watermelon Man - Herbie Hancock
I’ll Take You There - the Staple Singers
Can’t Get Enough of Your Love - Barry White
Annie’s Song - John Denver
I Shot the Sheriff - Bob Marley & the Wailers / Eric Clapton
Longer Than - Dan Fogelberg
Corner of the Sky - from “Pippin”
Rock & Roll All Night (and Party Every Day) - Kiss
You’re No Good - Linda Rondstadt
Knock Three Times On the Ceiling - Tony Orlando & Dawn
Walk On the Wild Side - Lou Reed
Come Sail Away - Styx
And also the theme songs to “Happy Days”, “Good Times”, “Wonder Woman”, “the Love Boat” and “Maude.”
To be fair, MTV began broadcasting in 1981 and changed its focus from videos to “reality shows” circa 2001, so their glory days draw heavily on the 80s and 90s.
Some songs so 70s-specific I doubt they’ve ever been pressed on CD format:
The Night Chicago Died (Paper Lace)
Beach Baby (Canadian Lighthouse)
The entire David Cassidy/Bobby Sherman oeuvre
One Tin Soldier (Coven)
The Streak (Ray Stevens)
Anything by Jim Stafford (Spiders & Snakes, My Girl Bill, Swamp Witch, Wildweed Flower)
Oh, yes they have. My mom bought one of those big CD collections off TV and she’s got all this stuff. (Which made it easy for my teenager to put it on her iPod).
Theme from Shaft (Isaac Hayes)
Freddie’s Dead (Curtis Mayfield)
Midnight Rider and In Memory of Elizabeth Reed (Allman Brothers Band)
Life During Wartime (Talking Heads)
And at least a few of the following songs from The Wailers/Bob Marley and The Wailers - and I’m leaving out plenty, dammit!:
Get Up, Stand Up
War
Exodus
Jamming
No Woman, No Cry
Lively Up Yourself
Them Belly Full
Concrete Jungle; Burnin’ and Lootin’
Zimbabwe
Africa Unite
Redemption Song [Uprising was released in 1980 but I think the song was written in 1979]
pretty much anything Stevie Wonder released
“What’s Going On” by Marvin Gaye
“Your Song” Elton John
“Bridge Over Troubled Water” Simon & Garfunkel
damn near every songs on “Rumors” by Fleetwood Mac
“London Calling” The Clash
“Piano Man” Billy Joel
Refugee (or American Girl) by Tom Petty and the Heartbreakers
“Sheena Is A Punk Rocker”, The Ramones
“Anarchy in the U.K.”, Sex Pistols
“At Home He’s A Tourist”, Gang Of Four
“Roadrunner”, The Modern Lovers
“Flashlight”, Parliament
“Statue Of Liberty” and “Making Plans For Nigel”, XTC
“September Gurls”, Big Star
“Surrender”, Cheap Trick
“Superstition”, Stevie Wonder
“Let It Be”, The Beatles
“White Man In Hammersmith Palais”, The Clash
“Dreaming”, Blondie
“(What’s So Funny 'Bout) Peace, Love & Understanding?”, Elvis Costello & The Attractions
“Cruel To Be Kind”, Nick Lowe
“New Rose”, The Damned
“Tired of Waking Up Tired”, The Diodes
“Me and Julio Down By The Schoolyard”, Paul Simon
“Sweet Jane”, The Velvet Underground
“Let’s Go”, The Cars
“I Don’t Mind”, Buzzcocks
“Don’t Stop 'Til You Get Enough”, Michael Jackson
“Tusk”, Fleetwood Mac
“A Message To You Rudy”, The Specials
Hey! Springsteen wrote it and released it first. Mann’s version was just more commercially successful.
I’d add “Darkness on the Edge of Town” and Rosilita by Springsteen.
Somebody ought to add some Fleetwood Mac, Led Zepplin, and Aerosmith to those listed here. Given that “Stairway to Heaven” is already on here, how about,
“Walk this Way” for Aerosmith
“Immigrant Song” (Hammer of the Gods) for Led Zepplin (C’mon guys, they must have 100 all by themselves, include something besides “Stairway to Heaven”.)
“You Make Loving Fun”, “Over My Head”, “The Chain”, “Tusk” Fleetwood Mac.
Also,
LA Woman - The Doors
I was going to say “Call Me” by Blondie, but that appears to be 1980, how about, “One Way Or Another”?
A lot of mine are already listed. Here are a few more:
Papa Was a Rolling Stone - The Temptations
Thunder Road - Bruce Springsteen
I’ll Be There - Jackson 5
Let’s Get It On - Marvin Gaye
She’s a Lady - Tom Jones
Brick House - The Commodores
Ooh Child - 5 Stairsteps
Brandy (You’re a Fine Girl) - Looking Glass
Truckin’ - Grateful Dead
Iron Man - Black Sabbath
After The Lovin’ - Englebert Humperdinck
Da Doo Run Run - cover by Sean Cassidy
Rock With You - Michael Jackson
The Hustle - Van McCoy
See post #24 above for both songs and all three groups.
Yeah, sorry. I started writing before those, but went to lunch before posting, and didn’t check what had been added.
Okay, we’ve listed well over two hundred. Now we need to cut out the duplicates, check qualifications, vote on worthiness, and tally the votes.
Y’all go ahead. 
(What’s So Funny ‘Bout) Peace, Love & Understanding?
A Message To You Rudy
Africa Unite
After The Lovin’
American Pie
Anarchy in the U.K.
Annie’s Song - John Denver
As, Stevie Wonder
At Home He’s A Tourist
Baba O’Riley, The Who
Back Stabbers - The O’Jays
Bad Bad Leroy Brown
Baker Street-- Gerry Rafferty
Band on the Run
Beach Baby (Canadian Lighthouse)
Bennie and the Jets
Best of My Love
Blinded by the Light
Bohemian Rhapsody
Boogie Oogie Oogie, Taste of Honey
Born to Run - Bruce Springsteen
Brandy (You’re a Fine Girl)
Brick House
Bridge Over Troubled Water
Brown Sugar
Burnin’ and Lootin’
Candle in the Wind
Can’t Get Enough of Your Love - Barry White
Carry on Wayward Son
Changes, David Bowie
Chevy Van
Cocaine
Come Sail Away - Styx
Copacabana
Corner of the Sky
Crazy On You
Cruel To Be Kind
Da Doo Run Run
Da Ya Think I’m Sexy, Rod Stewart
Dancing Queen - ABBA
Darkness on the Edge of Town
Delta Dawn
Desperado
Do You Feel Like I Do – Peter Frampton
Don’t Fear the Reaper
Don’t Leave Me This Way - Thelma Houston
Don’t Let the Sun Go Down on Me
Don’t Stop ‘Til You Get Enough
Dreaming
Dust in the Wind
Everybody Plays the Fool - The Main Ingredient
Evil Ways - Santana
Evil Woman
Exodus
Feel Like Making Love - Bad Company
Fire and Rain
Flashlight
Fly Like An Eagle – Steve Miller
Fooling Yourself, Styx
For All We Know
Frankenstien
Freddie’s Dead (Curtis Mayfield)
Free Bird - Lynyrd Skynyrd
Free Ride, The Edgar Winter Group
Freeway Jam - Jeff Beck
Get Up, Stand Up
Gloria - Patti Smith
Go Your Own Way, Fleetwood Mac
Good Times - Chic
Goodbye Yellow Brick Road
Horse With No Name
Hot Rod Lincoln - Commander Cody & His Lost Planet Airmen
Hotel California
I Am Woman
I Don’t Mind
I Need a Lover, John Cougar Mellencamp
I Saw the Light, Todd Rundgren
I Shot the Sheriff - Bob Marley & the Wailers / Eric Clapton
I Want You to Want Me, Cheap Trick
I Write the Songs
If You Don’t Know Me by Now - Harold Melvin and the Blue Notes
I’ll Be There
I’ll Take You There - the Staple Singers
I’m Not In Love - 10cc
Immigrant Song.
In Memory of Elizabeth Reed
Instant Karma - John Lennon
Iron Man
It’s Only Rock ‘n’ Roll, Rolling Stones
Jamming
Jive Talkin’, Bee Gees
Jungleland, Bruce Springsteen
Knock Three Times On the Ceiling - Tony Orlando & Dawn
LA Woman
Lady Marmalade, LaBelle
Layla – Derek and the Dominoes
Let It Be
Let’s Get It On
Let’s Go
Life During Wartime (Talking Heads)
Life in the Fast Lane
Lively Up Yourself
London Calling
Longer Than - Dan Fogelberg
Love Hurts – Nazareth
Love to Love You Baby - Donna Summer
Low Rider
Lucky Man – ELP
Lyin’ Eyes
Making Plans For Nigel
Me & Bobby McGee - Janis Joplin
Me and Julio Down By The Schoolyard
Meeting Across the River, Bruce Springsteen
Message in a Bottle, The Police
Midnight Rider
Midnight Train to Georgia, Gladys Knight & the Pips
Miracles - Jefferson Starship
Miss You
Mockingbird, James Taylor & Carly Simon
Money (Pink Floyd)
More than a Feeling – Boston
Mr. Bojangles, Nitty Gritty Dirt Band
My Girl Bill
My My Hey Hey
My Sweet Lord
New Kid in Town
New Rose
No Time Left for You – Guess Who
No Woman, No Cry
Ohio - Neil Young
One – Three Dog Knight
One Tin Soldier (Coven)
One Way Or Another
Ooh Child
Over My Head
Papa Was a Rolling Stone
Paradise by the Dashboard Light, Meat Loaf
Peaceful Easy Feeling
Piano Man
Play That Funky Music, Wild Cherry
Psycho Killer - Talking Heads
Put Your Hand in the Hand, The Ocean
Rainy Days & Mondays
Rapper’s Delight - The Sugarhill Gang
Rebel, David Bowie
Redemption Song
Reeling In the Years - Steely Dan
Refugee
Rhiannon - Fleetwood Mac
River Man, Nick Drake
Roadrunner
Rock and Roll All Nite, Kiss
Rock Lobster, The B-52’s
Rock ‘N’ Roll Fantasy, Bad Company
Rock With You
Rocket Man
Roll With the Changes, REO Speedwagon
Rosilita
Roundabout
Saturday in the Park-- Chicago
September Gurls
Sheena Is A Punk Rocker
She’s a Lady
Show Me the Way, Peter Frampton
Smoke on the Water - Deep Purple
Spiders & Snakes
Stairway to Heaven
Statue Of Liberty
Stayin’ Alive – Bee Gees
Sultans of Swing - Dire Straits
Superstar
Superstition
Surrender
Swamp Witch
Sweet Home Alabama
Sweet Jane
Take it Easy
Take it to the Limit
Takin’ It to the Streets, Doobie Brothers
Tequila Sunrise
Thank You (Falettinme Be Mice Self Elf Agin), Sly & the Family Stone
That’s the Way I Like It, K.C. & the Sunshine Band
The Ballroom Blitz, Sweet
The Best of My Love, The Eagles
The Boys Are Back in Town, Thin Lizzy
The Chain
The Hustle
The Low Spark of High Heeled Boys
The Night Chicago Died (Paper Lace)
The Streak (Ray Stevens)
The Time Warp, Rocky Horror Picture Show
Them Belly Full
Theme from Shaft (Isaac Hayes)
Thunder Road
Tired of Waking Up Tired
Top of the World
Treat Her Like a Lady - Cornelius Brothers and Sister Rose
Truckin’
Turn the Beat Around, Vicky Sue Robinson
Tusk
Uncle Albert/Admiral Halsey, Paul & Linda McCartney
Ventura Highway - America
Walk On the Wild Side - Lou Reed
Walk this Way
War
Watermelon Man - Herbie Hancock
Weekend in New England
Werewolves of London
We’ve Only Just Begun
What’s Going On
White Man In Hammersmith Palais
Wildweed Flower
Witchy Woman
Won’t Get Fooled Again
Y.M.C.A.
You Are the Sunshine of My Life, Stevie Wonder
You Make Loving Fun
You Sexy Thing - Hot Chocolate
Young Americans, David Bowie
Your Mama Don’t Dance – Loggins and Messina
Your Move, Yes
Your Song
You’re No Good - Linda Rondstadt
Zimbabwe
“Let My Love Open the Door” – Pete Townshend