Quote: I'm with the "depends on my mood" opinion. Today it would be one of these:
Amazing Grace Can't You Hear Me Knockin Low Spark of High Heeled Boys Me and Bobby McGee
Amazing thing about Amazing Grace - It can be sung or played in any musical genre and still dig deeply into the soul. I agree with Moe and Paula that there probably isn't a 'best' but it doesn't get much better than A.G. - Good music with a good message - and that seems key to what usually defines a good song - both the notes and the lyrics. Nearly all that have both become timeless.
I'll add to the list anything sung by James Taylor, Alison Krause, and Sarah McLachlin and most C,S,N &Y, particularly the numbers composed by Stills - All head music (I ain't much for dancin' - Like to listen).