Someone already mentioned Barbara Mandrell, these were a few of my favorites..
On a CD of hers that I have, she said she got a letter from a prisoner who said he loved his wife very much, but would never be with her again. He said he liked this song. (It really puts it in a sad and different light from that perspective).
Someone mentioned Tammy Wynette, and yes, she was without a doubt one of the Top 5 female singers of all time. It is, in a way, unfair to compare the singers from the early years of country music to the singers of the later years, because of the way country music has changed. Take Kitty Wells, for instance, and her style, versus Miranda Lambert of today. In my opinion, Wells was just as good because I prefer the music of the 40's-70's period as opposed to today's crap. "It Wasn't God Who Made Honky Tonk Angels" is a country classic, and Kitty Wells is also the number 6 ranked country female singer of all time according to Billboard's magazine rankings.
None of the women of today, or yesterday, will ever be as good as Patsy Cline. If you don't agree with that, then you either don't listen to country music, or else you just want to be different. Lorretta Lynn, Wanda Jackson, Connie Smith, Skeeter Davis, Jean Shepherd, Melba Montgomery, Anita Carter, Norma Jean, Tanya Tucker......the list could go on, but these were all gals who actually sang "country" music, not today's crap that Nashville is trying to pass off as being "country."
Emmylou Harris for me. Breaks my old, wicked heart. Kills me that she was performing regularly in local clubs when I was young, and I was too dumb to know it.
More bluegrass than country, but Alison Krauss is amazing, and what a band. Another bluegrass gal that's coming on and is a killer musician as well is Sierra Hull.
Rachel Potter is a fantastic singer who is wasting her talent IMHO singing modern country (which I detest for the most part). She's got tons of stuff on YouTube. That girls's been in a couple of Broadway shows and can literally sing anything well. Hot little number too.