Woodstock. By Joni Mitchell. Also performed by James Taylor, Crosby Stills Nash and Young, and Mathews Southern Comfort.
It doesn't even matter which version.
Whenever I hear that song it reminds me of what we as a society have lost, and what we could have become. I can't even sing along, because my voice cracks every time.
Also... I Had a dream, by John Sebastan. Also performed by Joss Stone. Again, either version will do...

I had a dream last night
What a lovely dream it was
I dreamed we all were alright
Happy in a land of Oz

Why did everybody laugh
when I told them my dream
I guess they all were so far
from that kind of scene
Feeling mean

I heard a song last night
What a lovely song it was
I thought I´d hum it all night
Unforgettable because

All of the players were playing together
And all of the heavies were light as a feather
All I remember is a feeling tomorrow
And as I recall the rest will just follow

I had a dream last night
What a lovely dream it was
I dreamed we all were alright
Happy in a land of Oz
Happy in a land of Oz