Nicks or McVie.....

Stevie has a powerful dynamic voice, is an accomplished writer of evocotive lyrics and music, and was in her prime, an alluring stage presence...the whole package!  Christine's softer voice is more easy on the ears, but lacks the punch Stevie brings, and her songs don't carry the poetic weight of her bandmate's, IMO.  That they shared a stage and traded off in one of the greatest bands of all time...that's been a joy for all of us, so why worry who's "better"? 
As a pure singer, give me Linda Ronstadt any day.  Or Annie Lennox. Or Aretha. Or...

I like Julie London's voice. Never get tired of it. Yea, she is not rock and roll, but a beautiful soothing voice none the less.

Since we’re comparing them, we have to compare their work with Fleetwood Mac because Stevie definitely had the better solo career. When comparing their work with Fleetwood Mac, of their biggest hits, Stevie sang lead only on Rhiannon, Dreams, Sara, and Gypsy. Not bad, but Christine sang lead on 8 of their biggest hits, including my personal favorites, You Make Loving Fun and Everywhere. Got to go with Christine on this one. But her solo stuff was not so good. Oh, and as for greatest female rock singer, that has to go to Grace Slick in my humble opinion.

You don't suck at that level of success! Greatness befalls each of them.

McVie.

Especially on Bare Trees and Mystery To Me LPs......both prior to either Buckingham or Nicks.