It should be a song that is special to both of you, and one that, when you hear it years later, will send a thrill down your spines and give you moment of bliss. In my opinion, it should be between the both of you, not chosen by strangers.
Our was "Unchained Melody" - whenever we hear it now, we snuggle, squeeze hands, or dance, whichever is the most appropriate - and we have been married for 15.5 years.