There needs to be another thread where it’s not bad songs, cause this area is very well described now. It’s an embarrassment of riches.
It’s been done a few times.
Here.
Here.
This somewhat.
Here if you want to limit it to singles.
ETA: OK, 1,2, and 4 are about “best” songs. 3 is about what song best encapsulates the 70s.
First song that popped into my head after seeing the title was Song Sung Blue by Neil Diamond. I didn’t want it to, but there you go. Now sing along, errbody.
Chevy Van, Stayin’ Alive, and Afternoon Delight are my votes.
You’ve already mentioned all of the ones I immediately thought of, but a co-worker of my same vintage suggested “**Reunited (and it feels so good)” **by Peaches and Herb. I hadn’t seen that one mentioned and had to agree. Very 70s schlock.
Free Bird
There are many good songs mentioned in this thread!!
Manfred Mann’s Earth Band- Blinded by the Light (Complete with 70s cocaine montage from Blow)
There can be only one answer to this question, an answer researched by a man that not only lived through and survived the 70’s only to watch it devolve and blow up in our collective faces with the advent of disco. Only one song carries the banner of everything the 70’s represented, a song that captures the mood of an entire generation yet survives today only as a distant memory. Ladies and Gentlemen, I present to you for your approval, the one song that shouts out “I am the 70’s”…(drumroll please)…My Dingaling
^ You’re a brave soul; I thought of that two days ago and didn’t have the intestinal fortitude to post it. We are in your debt.
“Shoes” by Reparata. 
tl; dr
Convoy
American Pie
I have never understood the rather universal dislike of this song. It does rather drip with symbolism but so does a lot of music. I remember playing it for my kids and telling them that is Dumbledore singing.
And besides, without it we would never have had the absolutely fantastic tribute to the song in the Deleted Scenes of the DVD version of Starship Troopers (think Denise Richards in the park and a yellow cotton dress “on the ground around your knees”. 
For my votes to the op’s question I would have to go with any of…
- Old Fashioned Love Song - Three Dog Night
- American Pie (the song, not the movie) - Don Mclean
- Bridge Over Troubled Water - Simon and Garfunkel
All The Young Dudes - Mott The Hoople
“Mr. Jaws”…“Disco Star Wars” and “The Telephone Man” would like a word.
I was going to say “Come Together” by the Beatles, since one of my strongest memories was during a Cub Scout trip to Mystic Seaport, with my dad and the rest of my troop, and listening to that song for the first time on the radio. But I checked, and that was probably late 1969.
My second choice then would have to be “Kodachrome” by Paul Simon. There was something about that song that just brings me back to my childhood.
How about “The Streak”?
Blue Öyster Cult - “Don’t Fear The Reaper”
Not the only song that say’s 1970’s to me by any stretch of the imagination, but the one that seems to encapsulate the meaning of and what the 70’s were for me growing up as a young lad.
Unless I missed it - the winner is:
“Paradise by the Dashboard Light” is a song written by Jim Steinman. It was first released in 1977 on the album Bat Out of Hell, with vocals by the American musician Meat Loaf alongside Ellen Foley. The song is most notable for its unique structure and length, and has become a staple of classic rock radio
Tell Me Something Good - Rufus, featuring Chaka Khan