Chuck Negron, formerly of Three Dog Night, if you really take the time to listen to his versatility,
range, and pure beautiful sound.
The question was "greatest singer", not most influential or successful. So for pure vocal greatness, amazing vocal cords, wide octave range and perfect pitch, Chuck Negron is the best and under-rated. Most of his high notes are sung, not falsetto.
He did "ONE" in one take and also "Easy to be Hard" in one take.
He is still singing and has a great range. In concert, he does a long line-up and his voice is energetic, dramatic, and powerful all the way through. He's 67 years old, and has been singing publicly as an entertainer ever since he was 15.
The three part harmony he did with Three Dog Night blended magically. That also is a testiment to Chuck's talent as a great rock and roll singer. In some of the powerful songs they sounded like more that three guys singing. It was definitely some of the best rock and roll ever! The band took songs they knew would be hits and arranged them into unique master peices that showcased the vocals.
Thank you for this question!