The best last line of any song? For my money, it's from the same song whose middle eight contributed the melody to Leo Sayer's "When I Need You": "Famous Blue Raincoat," by Leonard Cohen. (This is also the song that my son Mark, who was six at the time, asked me to turn off because it was going to give him nightmares.) In case you've forgotten, that final line is:
"Sincerely, L. Cohen."