It's not surprising that Karen Carpenter's voice is so widely liked. It was supposed to be. What you would hear when she sang was actually not her true voice, but one that was augmented through the use of frequency modulation. Richard Carpenter was one of the first folks to recognize what modulating a voice could do, and was the one who drove the decision to use modulation to enhance Karen's voice. When you hear Karen Carpenter sing, you're hearing her natural voice plus a couple of higher and lower modulations, which is why her voice sounds so rich.
My vote for best female vocalist is hands down Celine Dion. I don't think anyone else comes close. She has a true four-octave range. Her first English-language album, Unison, wasn't sung in English. It was sung phonetically in French, which is extremely hard to pull off yet listening to songs from that album you would never know it. Before she recorded her next album she had learned how to speak English, and later learned how to speak Spanish so she could record in Spanish.
I know a lot of people got tired of Dion after My Heart Will Go On got played a gazillion times when Titanic came out, but there's a very interesting backstory to that song. James Cameron told composer James Horner that he wanted no lyrics at any point in the soundtrack, just instrumentals only. Horner wrote lyrics to My Heart Will Go On and really thought they would work great for the end of the film, but Cameron said no way. Right before the final recording session for the soundtrack, Horner calls Dion and asks her to fly to California to make a demo recording of the song that he could play for Cameron.
Dion flies to California, and Horner hands her the lyrics and has the orchestra play through the song one time while Dion reads through the lyrics. They then recorded one pass of her singing the song that Horner could take to Cameron. Cameron objects, but ends up listening to the track and says he wants it in the movie. A relieved Horner says great, we'll record a better version and Cameron says no, I want that one just as it is. So when you hear that song played at the end of the movie, that's Dion singing the song cold for the first time, with no practice, after just getting off a flight from Canada, and having only heard it one time. I can't think of another singer, male or female, who could have pulled that off.
