Wow...depends on the taste of the couple, I would think. Some people want traditional and go with "Ave Maria". Some people want contemporary and want something like "The Wedding Song". Some people want classical and like "Pacelbels Canon". Some people want doo wop (yup...I sang doo wop *during* a ceremony for a friend).

So, find out what they want for their wedding, and then tailor the music from there.