Which lines from songs are among the saddest?

One that I would suggest is this, from 'Changes' by Phil Ochs:

"And I'll kiss you one more time,
And leave you on the rolling river shores of changes."

And another, from Don McLean's 'Empty Chairs':

"And I wonder if you know,
That I never understood,
That although you said you'd go,
Until you did, I never thought you would."

And, from 'Softly as I leave You', sung by Matt Munro and Frank Sinatra (and others):

"Softly, I will leave you, softly,
For my heart would break, if you should wake and see me go."