Tony Bennett. Always was more of stylist than crooner. Limited range. Ah, but what an entertainer, total professional.
Margaretville? 
Willie Nelson might make it through a round or two before being cut on grounds his voice is too nasal.
Yeah, that’s more like what I got from the thread title as well.
I stumbled over that as well and also went 
Mariah Carey is a notoriously poor singer live. Hence the recent Rockefeller Center incident. She failed to prerecord her song, sang it live, somebody in the sound booth made a copy of just her voice track and released it on the Net. It is awful.
Whenever she has problems with her taped vocals, she suddenly gets a case of “I ate bad clams.” and concerts are postponed until it’s fixed.
Stevie Nicks also did a poorish job live. But it’s still pretty good for the type of songs she sings. Put her on a singing show where she has to sing songs live outside her zone and it will not go good.
Not one of my favorites would stand a chance. Kate Bush, Happy Rhodes, Jane Siberry, Noe Venable, Siouxsie Sioux, Sinead O’Connor, Mary Margarett O’Hara, Liz Fraser, Lisa Gerrard, Mary Coughlan, Azam Ali, I could go on and on and on. All have amazing voices to me, and not a single one of them would have made it out of the regional rounds, because they don’t have that impressive but (IMO) boring “Power Voice” that seems to be required.
To me, that kind of Power voice is only impressive in 8-year olds, which is why I love watching “kid singer” videos of singers like Christina Agulera, Britney Spears, Shakira, and others, but would never watch/listen to them as adults. I’m not meaning to say they’re bad or anything, it’s just that that type of voice and the music that goes along with it is not interesting to me personally.