1. I think a lot straight rock vocalists would work as prog vocalists.
2. Steve Perry, despite VAX's opinion (!) is perhaps the greatest tenor ever in rock music. He didn't ruin Journey (at first); Jonathan Cain ruined Journey (or it could be argued Gregg Rolie ruined Journey when he left!). Even with Cain, Journey released two great albums (Escape, Frontiers), but THEN Perry killed them by pushing them to make an R&B album (Craised on Radio). Cain wrote the great "Faithfully," but first "Open Arms," which while assuring them their retirement, put them on the path to try and recapture that EVERY ALBUM SINCE. I don't like you, Cain.
3. Someone with lots of vocal versatility, like Bono, would be a great prog vox.
4. Best pure rock vocalist if you just want to rock out? Eddie Vedder of Pearl Jam. Check out "Alive"
http://www.youtube.com/watch?v=e_LQU2-GOXY
As a prog vocalist? Maybe in a prog metal band.
5. Elvis Presley, anyone? :P
6. Though I doubt anyone here's heard of him, Pete Stewart, the guitarist and singer for '90s Christian metal/grunge band Grammatrain has an amazing Rock voice, but I don't know it it'd translate to prog.
My six cents.