If you sit down with Rumours, and really give it a listen ... the voice that most folks associate directly with the sound of FM is Christine's voice.
Whether she's singing with Buckingham or Nicks (or both of them), her voice is the solid anchor for almost all the ensemble songs in their repertoire.
Sure there's the Nicks solos, and the Buckingham/Nicks mega-hits (but even they always have a bit of Christine in the background) ... but Ms. Perfect was the anchor voice of Fleewood Mac.
Interesting side-note to the mega-successful Rumours is that the album was engineered by Ken Caillat, whose daughter happens to be the brilliant Colby Caillat, with two of her own Grammys, six million albums sold, and ten million singles sold (all produced and engineered by her dad, as you would expect).