There is a new documentary premiering at the Sundance Film Festival called 'Twenty Five Feet From Stardom" about the art of singing behind the main act. Included are Darlene Love & The Blossoms, Merry Clayton, Patti Austin and many others.
I have long maintained that solo artist records should not just say "Joe Blow", but "Joe Blow and a bunch of really good people you've never heard of before" and these folks should be credited for the work they did to help make "Joe Blow" a star. To that extent, I once wasted a ton of bandwidth on a blog about the contribution The Young Sisters made to Del Shannon's "Little Town Flirt".
So I guess the question in front of the masses is do you have a favorite set of back-up singers, or one particular performance that makes (or breaks) a recorded performance?