I mean in terms of making the hairs stand up on the back of your neck, I mean in terms of achieving a quite ludricrously high pitched note and holding it for a quite ludicrously lengthy period of time. Ultimately, I mean in terms of simply having an awesome singing voice. I can admit that some of her work has been criminal but when she sings something worth singing to the best of her ability, there is quite simply no one that can touch her.
Boy George doesn't come close for me and I must admit that I'm not that familiar with Jeff Buckley's work. The only time I can ever remember having heard anywhere near such an impressive vocal performance is Mini Ripperton's Loving You. A fantastic song, sung beautifully. She's still behind Mariah Carey for me though.