I said singer, should have said artist. Gwen is a resounding no. Mariah better singer, but 20 years would be a push at worst. Christina is an argument worth taking that side.
The rest of your list:
Britney (um, fuck no, that's manufactured pop right there)
Rihanna (not American, part of my argument)
Celine (Canadian, making my point)
Mary J (good artist, but no Pink)
Shania (again Canadian, you are killing me)
Beyonce (she's hot, black, and thick, but watch Pink's grammy shit and say good night)
Gwen (sells purses well, lol)