Kelly's voice has only improved substantially, an example of this is her belting range improving to being the largest of any of these four vocalists (B4 - C6). Her belts have become more resonant and what I've seen from the other three, she is the only one able to access the upper parts of her belting range without having to scale up to it (look at the last belted A5's in
prollyallmyfault's video posted on this page).
Kelly's tone has also improved in that it's become more soulful (see Please Come Home For Christmas) and warm (see Have Yourself A Merry Little Christmas). Kelly can also
morph her tone to whatever tone she needs it to be. In rockier songs she may implement a raspy sound and then immediately switch to a butter-smooth tone for a more R&B song. She is the best singer I've
ever seen at manipulation of tone.
Out of all these vocalists, Kelly's voice sounds completely different but in a great way as her voice has improved by leaps and bounds. I can barely listen to her first album because her voice is nowhere near as good as it is now.
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Beyonce and P!nk have done a good job keeping their voice in good shape.
Christina on the other hand...
If you can't tell, my vote goes to Kelly!