I'm not one for love songs, but if I were pushed for a couple I'd say:
The Flamingos - I Only Have Eyes for You
The Beach Boys - God Only Knows
Al Green - Let's Stay Together
Etta James - At Last
Elton John - Your Song
Paul McCartney - Maybe I'm Amazed
Roberta Flack - The First Time I Ever Saw Your Face
John Denver - Annie's Song
Bill Withers - Ain't No Sunshine
Harry Nilsson - Without You (I know it's a Badfinger track, but Harry's was better).
The Flamingos - I Only Have Eyes for You
The Beach Boys - God Only Knows
Al Green - Let's Stay Together
Etta James - At Last
Elton John - Your Song
Paul McCartney - Maybe I'm Amazed
Roberta Flack - The First Time I Ever Saw Your Face
John Denver - Annie's Song
Bill Withers - Ain't No Sunshine
Harry Nilsson - Without You (I know it's a Badfinger track, but Harry's was better).