Liked many of the choices, but so subjective, of course, and many ways to bend it (best now? best at their peak? best over a long haul? best at a style? best singing anything?...)
So, I'll subvert the question to "Top Three Male Progrock Vocalists", and put up my contenders:
Greg Lake (already mentioned—not only ELP, but the fact that In the Court of the Crimson King was the blueprint for prog)
Peter Gabriel (can we get more prog than early Genesis? With more vocal experimentation?)
John Wetton (impressive body of work over many bands, and what a voice)
Honorable mention to Jon Anderson. All these guys are instantly identifiable, even if they sang another style completely.
So, I'll subvert the question to "Top Three Male Progrock Vocalists", and put up my contenders:
Greg Lake (already mentioned—not only ELP, but the fact that In the Court of the Crimson King was the blueprint for prog)
Peter Gabriel (can we get more prog than early Genesis? With more vocal experimentation?)
John Wetton (impressive body of work over many bands, and what a voice)
Honorable mention to Jon Anderson. All these guys are instantly identifiable, even if they sang another style completely.