I have produced quite a few wedding projects,I usually ask the couple themselves (Is it your wedding ?) and have been requested to add tracks including :
Angels : Robbie Williams
When You Say Nothing At All : Ronan Keating
The Power Of Love : Jennifer Rush
Your Beautiful : James Blunt
Show Me Heaven : Maria Mckee
My Heart Will Go On : Celine Dion
Strangest request I had was The Darkness : I Believe In A Thing Called Love .I thought err... ok if thats what you want !.Actually went quite well in finished project and they loved it !!!.
Watched a wedding slideshow some tool had set to Kenny Rogers/Dolly Partons : Islands In The Stream which was cheesy and pants beyond belief.More to point the couple HATED IT !!!.Got me to redo it to music of there own choice.
Music is a very personal choice if the slideshow if for someone else I would ask them for there choice and maybe give them a few examples.Just remember you are supposed to obtain a copyright licence to reproduce copyrighted work.However,If your after royalty free pieces have a look here.I have used some of Kevin Macleods pieces in a few of my projects.There are very some very good professional pieces to be found on his site.Just read the rules before using any.He asks for a $5.00 donation and to be credited for his work in your project.Other than that they are all free to download and try.His site is
http://incompetech.com/
Hope this helps.