I always find these rock lists kind of strange, because if you ask this question of other genres you will get what most people, including vocal coaches, consider "good" - like phrasing, range, pitch, smoothness, etc. But in rock I see all kinds of names on these lists who are far from good in most of these categories. As Stephen Tyler proved, most of these singers couldn't sing The Star-Spangled Banner if their lives depended upon it. They might fit their style of music, and I like a lot of them, but they don't have classically good voices. That said, I prefer the classically good voice that does satisfy these attributes.
Geoff Tate
Bruce Dickinson
Rob Halford
Messiah Marcolin
Steve Grimmett
Steve Perry
Bonnie Raitt
David Draiman
Ann Wilson
Ronnie James Dio