It pains me greatly to agree with Vol, but Hotel California along with Stairway to Heaven would be the best 2 songs of the 1970s. As for my favorites, I'll have to give that some more thought. Here's one that I think everyone should hear, partly because it's a really good song, but these young ladies are amazing!!!
As someone who really loves Stairway, I find it hard to say it's the best Zeppelin song, much less best song of the 70s. Of course best is matter of opinion in the end.
To me the "best" songs of the 70s (at least pertaining to the rock genre) need to span a number of bands.
Without digging too much some other songs that IMO in the mind of most would be close to them.
Go your own Way - Fleetwood Mac
London Calling - The Clash
Born to Run - The Boss
Blitzkrieg Bop - Ramones
Band on the Run - Wings
Imagine - John Lennon
Psycho Killer - Talking Heads
A number of songs by Pink Floyd
Superstition - Stevie Wonder
Highway to Hell - AC/DC
Paranoid - Black Sabbath
And that's staying in the Rock genre, without going to pop, disco, etc.
I think sadly the 70s were the last great decade for rock. Don't get me wrong, lots of great rock happened after that. But the 70s seemed to be for th emost part the end of bands who had the freedom of the Beatles. This is where I'm excited for the internet because it allows people more access to music than ever before so you dont' have to be a slave to a label anymore.