Pig and Donna were 50/50 to even sing in key. When they hit it, it was wonderful, and Pig poured everthing he had into it, but it seemed... affected at times. At least to me.
My nod goes to the Jerry/Bob/Brent combo.. preferably before Bobby went ballistic with the screeching and Jerry spaced the lyrics too much. Also, as Jerry got older (and perhaps as his health declined... emphysema??), his voice gained a deep gravitas that I thought lent added emotion to tunes like Brown-Eyed, Brokedown, China Doll, &tc.
I personally think Brent was the best vocalist over the course of the band's run. But, having seen the Other Ones and The Dead a few times, it is Jerry's voice - even more than his guitar - that was missed.