Decided to answer with this 17-minute video worth every second:
There may be/have been better vocalists and/or sellers (though selling is sometimes VERY meaningless when it comes to quality) and it's common knowledge awards and charts have turned a blind eye to her smashing hits/albums too many times to count. But the energy and fierceness Britney has shown throughout the years is the definition of X-Factor. She does have the whole package: the looks, talent, attitude, charisma and that something else we can't exactly put into words.
Even Messy Jean has its good songs and yet something different she attempted at the time (WB was VERY unexpected from her back then, especially as a lead single), so there can only be added that her other records are a perfect mark of what Britney herself reinvented as high level pop in each era. Every single album has got at least one hit song (not necessarily #1 on BB, but definitely hit material, even GH:MP and TSC!
Musical production is impeccable and visual directing, just as much: her videography is almost immaculate (if it wasn't for Make Me (The Version That Worked for Larry Rudolph and Team Con). Her tours are top-notch incredible, having left a bunch of big industry names impressed, and more especifically when I watch the Intro and the Break the Ice Segue of TCSBS Tour I get so many shivers that I think I'm gonna have a seizure.
I never knew of another pop performer who could put on a show/performance quite like her: with back flips, fluid **** moves, sharp choreo + creative tech and the iconic hairflips (maybe Janet could jump too back then, someone please educate me (LOVE her but don't know that much about her older performances/tours...) Though I consider Janet more of an R&B star, despite various pop moments/elements. So yeah, I maintain my opinion re "female pop star").
Brinny is the real deal - as real as it gets -, she knows the industry inside-out more than 95% of other pop acts. Let's not forget she's always been called extremely professional by basically everyone she's worked with, down to earth and so humble that it's as if she had no idea the huge name she carries (which we now know has been true for a long time, unfortunately).
So, no. She is the one and only, Britney Spears