I agree with the Allison Krauss fans above -- her voice is simply incredible. As my dark horse pick, I would nominate Bonnie Raitt. Although she may not have the purest voice, she has to be one of the most distinctive female singers out there.
For jazz fans, I would strongly second Rayhall's recommendation of Dee Dee Bridgewater -- Live at Yoshi's. And if you like Bridgewater, try Mary Stallings, Live at the Village Vanguard. However, there's still no one more powerful than Billie Holiday.