If you are asking about something other than a religious song...how about, Wind Beneath my Wings?? If it is to be at the very end of the service or the very beginning, a good choice of a religious song is the Lords Prayer. I would say good luck, but it is obvious, if you have been asked to sing at such an important event you must have a good voice and are talented...good luck is not needed if you have skill and talent.
"Rest In Pieces" by Misanthrope is always a good choice...
Everyting I do I do it for You by Bryan Adams is a good wedding song.
Here comes the bride