It's all about range and tone. Consistency in performance and overall health help too.
I don't respect a guy (or gal) who can sing great in the studio but totally bombs in live performances.
Bruce Dickinson manages both aspects very well. He has a great depth and solid range, live and in the studio. The lead singer of Queensrche (Geoff Tate???) has some great depth and range also. He doesnt have the professionalism of Bruce; nevertheless, his studio stuff is phenomenal.
"Eyes of a stranger"
"I don't believe in love"
"Jet City woman"-- my favorites.
Dream Theater: "Pull me under" and "another day" showed some crazy vocal talent. Not sure who was doing the vocals. Petrucci seemed to be the only one who really rose from the ashes.
Joe Lyn Turner with Rainbow was kickin' for a while. Not sure where he went or what happened.
Sebastian Bach and Axel Rose had their moments, but only in the studio. Live performances aren't even worth mentioning. Voices were trash by the time they were a week into their tours.