There's a difference between "favorite singer" and "favorite overall musician" so I'm going to focus on vocal power. This includes those who may not have a technically "beautiful" voice with lots of range, but contain enough emotion in their vocals to lift the song to the next level. I'm certain I've forgotten some but this is off the top of my head (just for kicks I linked to some examples of their vocal prowess).

1. Stevie Nicks
2. Glenn Frey
 3. Sarah McLachlan
4. Don Henley
5. Bruce Springsteen
6. Bono
7. Whitney Houston
8. Bette Midler
9. Billy Joel
10. Lindsey Buckingham

Honorable mention:
Ladies: Aretha Franklin, Nancy Wilson, Grace Slick, Linda Ronstadt, Kelly Clarkson, Roberta Flack, Amy Lee, Jennifer Hudson, Annie Lennox, Trisha Yearwood, Ella Fitzgerald, Celine Dion, Christine McVie, Chrissie Hynde
Guys: Nat King Cole, George Michael, Paul McCartney, Josh Groban, Phil Collins, Chris Isaak, JD Souther, Jackson Browne, Bob Seger, George Strait, Dan Fogelberg, Joe Walsh, Timothy B. Schmit, Randy Meisner