Van Morrison and Roy Orbison had pretty good pipes.
I'll throw in Lowell George to keep the turtle happy.
Lowell was awesome, especially as he got towards the end, his voice grew deeper and more commanding, a progression not unlike Elvis P. or Jim Morrisson.
I would like to see Roy Orbison and Chris Isaak in a sing-off. Both have nearly unmatched range and clarity. Octave after octave after octave.....
I have to take exception to Natsaddict's inclusion of Jackson Browne. He has very little range, really struggles to get where he needs to be, though he conveys his meaning fairly well. Don't get me wrong, I am a huge fan of Browne's early stuff in particular (Late for the Sky, for example)- he is a superior songwriter and belongs in that category as a great artist, but not for voice in my opinion.