"Out of the Woods" is the popular and most sensible choice. It is already dominating as a promo single and the track itself is very original, authentic, dramatic, lyrically superb and catchy.
"Welcome to New York" has simply set the tone for the album, which was pretty clever, so I believe any teen-pop song on the album such as "How You Get The Girl", "All You Had to Do Was Stay", "I Wish You Would" would be suitable for a future single.
"Bad Blood" is different, controversial and terribly good (more of a back handed compliment), it is also comical when you consider the background of the song. So that'd be a great single contender.
