I could name half a dozen singers you’ve never heard of before who make her sound like shit in contrast (like Maaya Sakamoto), or I could just say “everyone not her,” which works because it’s true.
I could care less about the range of her vocals if she uses them poorly. If I made a profession out of whining like she does I could hit those notes too.
A Kazoo has “tremendous range.” That doesn’t mean it sounds good. Ever hear bagpipes? They can hit higher notes than Christina can. Is it pleasant? You decide. She’s simply not gifted with a pleasant voice. She may not feel any strain hitting those notes, but my ears sure fucking do, and the synthetic electronic noise she sings over doesn’t help to flatter her skills either.
Hell, Christina’s not the only one with this problem. Ayumi Hamasaki has a far higher range than her, but still sounds terrible at some points because she just can’t create a soft, beautiful, singing voice like, say, Noriko Mitose can. Her voice is just too sharp, like needles in my ears.
The reason I’m using japanese singers for comparison is because frankly, there aren’t many non-shitty female english singers out there. I can only think of a few, like Sarah Brightman (she sang Dance of the Diva from The Fifth Element. Just wow.) and Sheryl Crow.