The three that come to mind for me have all been mentioned.
Pink - if anyone doubts her as a vocalist, Youtube some of her acoustic performances of Perfect and Who Knew. She is phenomenal.
Kelly Clarkson - I think she gets more credit from her peers and fellow singers than she does from the general public, outside of her fanbase. She can literally sing anything, even that proverbial phonebook everyone goes on about.
Nicole Scherzinger - she has an outstanding voice. I think people don't pay attention to it as The Pussycat Dolls music wasn't exactly groundbreaking, and her solo career hasn't been as succesful as it could be. But songs such as ArmenJena and Run showcase her voice excellently. I heard an audio of her performaing Memory, and it sounding amazing! I'm seeing her in it next month, I can't wait!
Another one that came to mind is JC Chasez from NSync. His voice is amazing, and he is by far the more talented singer and musician, in my opinion, out of him and Justin.