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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


One of the best concerts I ever saw (and I've seen just about them all) was Meatloaf at Central Park Bandshell. He did Paradise by the Dashboard Light with the chick from the album and Phil Rizzuto.
 
Steve Marriott
Lou Gramm
Bon Scott
Jimmy Barnes

Steve Marriott (may he rest in peace) had an amazing voice. I was lucky enough the see Humble Pie at the Filmore East when they were at their peak. Loved him in the Faces as well.
Bon Scott's (may he rest in peace) voice wasn't technically good, but his style was perfect for AC-DC. Much better than Brian Johnson's IMO..
Lou Gramm had/has? a great voice. To be honest, I was never a fan of Foreigner..
 
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