This is really silly. I'm an old sentimentalist at heart. You'd think my favorite love song would be something like "Singing Bird," "Flow Gently Sweet Afton," "My Bonnie Dearie (Ca' the Yowes...)". Maybe the Fairport Convention setting of "to Althea from Prison." And all of those are GREAT love songs in my book.   

...But for true depth of feeling -- heart-stopping, gut-wrenching, "I'd lay down my life for you" love -- the song that first comes to mind is Richard Thompson's "1952 Vincent Black Lightning," a song about two people I wouldn't want to meet in an alley and whose primary connection seems to be a motorcycle. Go figure.