Agreeing with some and just thought of some more ^^
Oh yeah, before Whitney Houston tore herself up, she had an amazing voice too
A good vocalist would ideally, to me, be someone like Freddie, because his voice went about 2 octaves I think and he could switch really fast between keys, plus he just NEVER messed up live. He was the perfect singer. But in reality there are a lot of different ways to sing, different techniques and some of them doesn't appeal to everyone. I can't stand Celine Dion's voice for example, but she is a fantastic vocalist, no doubt.
Regardless of their personalities (from what I've heard they're both kinda...odd) Mariah Carey and Christina Aguilera are both really talented vocalists as well.
I think the faeroese singer Eivør Pálsdóttir has got some serious voice control/talent as well. She can go vibrato, she can go high, low, she can go whispery (is that a technical term? lol, sounds like I don't know wth I'm talking about xD) Again, singing styles may not appeal to everyone, but you can't deny she's an amazing singer.