I'm having a hard time doing this, because I would just pick out my favorite singers and say they could sing anything. Prog really isn't different, unless you can yodel or go "cha cha"
But here's an interesting thought: Ayreon has all kinds of different singers on each record, many non-prog (many are though). An example is Damian Wilson, who I think has a killer voice (His performance on "And the Druids Turn To Stone" is breathtaking). Another example of the genius of Arjen Lucassen.
Actually Barry Manilow could work....
