Wow. That is the best question I’ve ever heard here b/c it’s such a hard decision.
I adore Etta James. (Damn your eyes, her version, makes me bawl.)
but Koko Taylor has some pipes that are amazing.
I’d have to go Etta at her best. But there is no loser in this.
Koko, without a doubt. And yes, I’ve seen both of these incendiary ladies perform live. Etta’s real good, don’t get me wrong, and her joy in performing for the audience is pure and humonstrously evident. But Koko…is…mighty. She’s truly awesome, in the sense that her onstage presence inspires awe. Heartbreak, heat and a touch of downright fear, walking like a woman, singing like a thunderstorm goddess traveling semi-incognito.
“Tell Mama”. “Beer Bottle Boogie”. “Come to Mama”. “Thank You, But…” . “I’m A Woman”. The lady’s got Heaven and Hell in her mouth and the four winds in her throat. And that’s all that one can say on the matter, really.
She was at the HOt Springs Blues festival this year, but I didn’t have money to go.
Don’t know Koko Taylor’s work at all* but I have to say the live version of “Baby,What You Want Me To Do” by Etta James gives me shivers every time I hear it. 
I’ve been in a sassy blues mood lately, and I would like to thank the OP for starting this thread.
I have to say watching a youtube video of a black woman with pipes wearing a blond wig, nordic ski sweater and eyeline like that would be such a put off, but her voice… damn.
I don’t know anything about Koko Taylor, but I would like to contribute my 0.02 worth of story about Etta James. There have been a couple of times when I would be listening to the radio while driving and I would hear some music and not even know who the artist was and there would be something about it that would make me take notice and say, “Damn, there’s something special about them. I wonder who that is?”
Once, it was Wynton Marsalas playing a Hayden trumpet concerto. Another time, it was Etta James singing “At Last”. It made an impression. I bought a CD just for that song. Unfortunately, there was a lot of pop tripe on the rest of the CD, so I would love to hear recommendations for more bluesy mood recordings.
Blues to the Bone caused me to start this thread.
Sorry but I can’t. This is a music genre that I don’t have down as well as some others yet and I’m still hunting down the essentials for myself (essentials as determined by me, of course YMMV). I’ll let you know if I come up with anything though.