Adele has a lovely, pure tone and her voice effortlessly oozes emotion. She's a great vocalist because although her range is extremely limited, she knows exactly how to use it.
Christina has more natural talent than both women - her tone is rich and incredible, she has a great range and a super head voice (not quite whistle). But she's arrogant and has never sought to improve and to smooth out the countless flaws in her voice. The result is that her technique is bad and has gotten progressively worse with every year - she screams out her head voice and belts (the latter of which should be mixed), her diction is slurred, her runs are heavy and overdone etc. She has an incredible voice but she's a bad, bad vocalist.
Beyonce is the clear winner. She doesn't have as much natural talent as Christina, but she has worked tirelessly on improving herself as a vocalist and she is always improving. She's a great all-around singer with an accomplished lower, mid, belting and head voice. My biggest critique of her was her tone, its thinness and the fact that she often attempted to cover up her lack of soul with vibrato, but she really found her true voice on 4 and Soul was dripping from her vocals.
JoJo will be better than all, though.
