It's not just a song, it's YOUR song. The song you'll hear years from now and instantly feel nostalgic. It's the song that will fill you with happiness as you reflect on how it made you feel as you danced with your beloved for the first time as newlyweds.
There are a lot of great songs out there; some old, some new. The tip I give to all couples trying to find THE song is, find a song that is special and meaningful to you both and that alone will transcend the trends of music overtime.
Here are a few first dance worthy songs, in my opinion...
John Legend - All Of Me
Sam Smith - Latch (Acoustic)
Van Morrison - Into The Mystic
Norah Jones - Come Away With Me