I'll be the first (of what I assume will be many votes in support) to mention Eva Cassidy. Purely a singer (as far as I know), her voice is simply haunting and beautiful. I'm usually a big lyrics guy. Most songs I love I gravitate towards because of the lyrics. If I like a song because of it's melody, it's the melody of the song and not necessarily the singers voice (in fact, I can't think of any examples). With Eva, all the love is in the voice.
Fields of Gold
Over the Rainbow
I'm also a huge fan of
Diana Krall. I'm not doing that great at naming singers/songwriters, as I'm pretty sure Diana doesn't write much of her own stuff (at least on her older recordings). I played piano forever and everything I did was classical. When I saw Diana play jazz piano, god, ten years ago when I was in high school, it made me extremely jealous and pissed that my teachers never opened the door to jazz. Specifically,
this song. That got me to listen to her recordings, and her vocals just blow me away. Not gonna lie, most of it is because I think her voice is sexy as hell. Plus, her version of Joni Mitchell's
A Case of You is defintely in my top ten songs to listen to.
If we were focusing on songwriting and not singing (or singing/songwriting), Joni Mitchell would be number one, but while I can stand her voice, it isn't my fav.