Wow.. there's quite a bit to the ol' greatest vocalist category. I judge it by the chicken skin factor.. some of my favorite vocalists that rank high on my chicken skin meter:
Amy Hanaiali'i (Kuakana Waialua A'oMoloka'i Live) IZ Uncle Cyril Uncle Dennis Makaha Sons Hoku Auntie Haunani Apoliona
Mariah Carey (Oh Holy Night) David Archuleta (Imagine Live on Idol) Love Willie Nelson & Johnny Cash too!
When it comes to the golden voice, for me it's Australian country artist Lee Kernaghan. What a wonderful, rich baritone. I could listen to Lee all day and never get tired of his angelic voice. BTW - this week he just got his 27th Number One song and was recently named Australian of the Year. He makes me proud to be a fan.
I will agree that Ralph Stanley is a fabulous bluegrass tenor. Been listening to his music for years.
I have heard Robert for over 40 years and love his voice. It is very distinctive and beautiful. I don't feel a need to compare him to other Hawaiian entertainers as I also love many of the other voices as well. In any case Robert is a standout!