I think it would be fair to talk about her peak, cause everyone's voice gets "tired" as they get older... And especially people like her, who's been singing high and low and better than everyone else (IMO), of course her voice is damaged now. But she still can SANG.
Her music isn't really my cup of tea now, even though I admit I played out "Music Box" like a muthaf-kka way back when, but nobody can deny all the intricate harmonies she does, her tone and her legendary, often mimicked yet never duplicated head voice (known as whistle note). For all of that, I'll give her a well-deserved 10.
EDIT: That paragraph was about Mariah, of course.
This thread moves really fast. Taylor - 3, either live or recorded.
A mess.
I do not find her tone as amazing as some people do, but she was an amazing vocalist.