People call me nuts, but my favorite is Billy Corgan. He isn't "good" per se in the usual sense of the term, but it fits his style(s) of music like no other voice could.
In no particular order:
Robert Plant
Ann Wilson
Meat Loaf
Sebastian Bach
Amy Lee
Geoff Tate
David Coverdale
Bruce Dickinson
Steve Marriott
Lou Gramm
Bon Scott
Jimmy Barnes
Screamin' Jay Hawkins