Vocal Techniques
Preference is kind of subjective so I will be objective
1. Kelly Clarkson: No doubt and obvious number 1. She got it all, the quality, techniques, belting and even emotions. She is very dynamic and truly gifted vocalist. She's up in the same league with Celine, Mariah and Whitney as Simon said.
2. Jessica Sanchez: Having singing skills in such a young age of 16 years old is very impressive. What more if she turns 18 or even 20, so much potential. Quality and techniques also stage presence she got the confidence and knows how to work it. As Adam said "The Future Mega Pop Star".
3. Adam Lambert: The range and power this man has is no joke and amazing. Definitely one of the best. Can sing various of genres and sings those with ease! Among the men he's the best.
4. Jordin Sparks: She's a big vocalist and love the way she takes those big notes. Got the presence too and her timbre is very sweet yet powerful at the same time.
5. Carrie Underwood: She can sing the big notes and yet remains the quality of her tone. Although she may sing emotionless sometimes or doesnt get into the song but she's got big pipes.
in any order
Jennifer Hudson, Fantasia, David Cook, David Archuleta, Tamyra Gray.
and forget about personal bias, it's only a game.
Add J-Hud and make it a Top 6 and you pretty much nailed the task.
Carrie is not a good singer. She tightens up and chokes her throat hitting the high notes. She's really painful to listen to. I don't get why she won. 
but props to her quality or timbre for its stability and power still remains even when she goes high. Still she is like a singing robot watching her performances especially Alone and Making Love Out of Nothing at All I am not feeling her emotions. But those are subjective maybe some will agree some will not.
