If we're including ships as BNS is doing....
(also if I don't say who the song is by, assume its by Taylor Swift)
Out Of The Woods and Wonderland are fitting for 5A Captain Swan with the anxiety in their relationship in that arc. For normal CS, I think Mine, Call It What You Want, King Of My Heart, State Of Grace, Begin Again and I'm A Pirate You're  A Princess by (I think) coldplay, as well as, obviously, The Words by Christina Perri.Â
I think Dear John, All Too Well and Better Man by Little Big Town are good for post s4 RumBelle.
I think Delicate is a good Curious Archer song- "my reputation's never been worse so/you must like me for me".