There's no way to point to just one song, but for the time being I think "Slow dancing in a burning room" by John Mayer is a great song. The title alone is a beautiful metaphor for a love going down and we don't too much af that these days. The lyrics and the music go very well together too.
I thought of a Beatles tune as well, and I'll second whoever wrote "A day in the life".
Sting has written a lot of great songs as well, both lyrics and music wise.