There is no greatest song.
But mostly, music keeps evolving and through that, keeps getting better. People learning from mistakes, or taking the previous and expanding on it.
But... it's art and therefore subjective. Most cannot divorce music from its context. From the time it was recorded right through to the songs you got laid to.
It's much more than the songs themselves that make them "great".
One person will hear a mess of an arrangement, an embarrasement of lyrics.... and another will hear the soundtrack of when life was better.
And call it "great."
But the song itself? Broken down and analyzed.... structure, musicianship, production, arrangement, lyrics, recording and on and on... often reduces it to something far less than what it wound up becoming.
That's where the context comes in.
Simply put, when folks say "Freebird" they aren't saying that the song is great, so much as they're saying "Man, those were good times."