When a champion mare retires, the whole world has an opinion about which stallion she should be sent to.  The romantics would send her to another recently retired champion, eg Frankel or Pierro, the two horses that are getting the most media attention.
With a background in science, I prefer a more rational approach.

Full article here: http://www.breedingracing.com/feature-files/2013/05/02/expert-reveals-best-sires-for-black-caviar/

Footnote:  Black Caviar’s owners decided on Exceed and Excel for her first mating. This filly has been named Oscietra.