Ethel Merman was very loud and hit the notes, but I would not consider her one of the "greatest" voices, just one of the most famous.
I think my favorite voice to listen to would have to be Helen Gallagher. She had a very distinct sound with a lot of power and pizzazz, but could instantly flip to a softer and more nuanced sound. I just think the woman was nothing less than amazing.
One of the most impressive voices to grace the Broadway stage (as fleeting as it was) has to be Yma Sumac. My jaw drops every time I listen to Flahooley.
But overall, I think the "best" female voice has to go to Barbara Cook. Her voice displayed amazing range and character. I'm impressed with her in every cast recording she made.
For men, I'm not sure anyone could beat John Raitt, but let's not forget the amazing range, emotion and clarity of David Carroll. Probably the best male voice on Broadway post-Golden Age.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian