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I did qualify Melchior by saying "almost" the greatest ...
I'd agree Bjorling is in there, and he had a very wide repertoir.
I've got a CD of Bjorling singing Donizetti, Verdi, Ponchielli, Gounoud, Bizet, Massenet, Mascagni and Puccini ... equally masterful in each role.
But !! ... for sheer ability to project his voice above the orchestra, without losing any vocal quality, I think Melchior has the edge.
There is a line somewhere between greatness and genius. Vocally, if you want to hear where greatness becomes genius, you have to listen to Kirsten Flagstad.
Maria Callas ... she was good ... no, she was great ...
... but simply not in the same class as Flagstad.