Forgive me but I'm not much of an expert on weddings but . . . I would imagine that it would have to be a ballad that would have personnal meaning between both you and the bride. A song that the lyrics would tell a story of how you both feel about each other (I know, mushy).
Album - Inside Job
Don Henley - On My Wedding Day
or
Album - Inside Job
Don Henley - Taking you home