GOOD topic here!
I concur with many that have already been mentioned here.
Of course, Jon would be at the TOP of my prog singer list . . . his voice is incomparable . . .
Linda, that long Finnish name is "Kuolema Tekee Taiteilijan" (English: "Death Makes an Artist"). And I concur heartily that Tarja has an amazing voice, and was a true asset to Nightwish . . . I love that Finnish song, and I have no clue what they're singing! But it's just SOOOO beautiful . . .
And I really like Anette too - when I was first introduced to Nightwish, it was with Dark Passion Play, and I was struck with similarities with Annie Haslam, who of course, would ALSO be on my top prog singer list . . .
And while we're on female singers, I like Heather Findlay as well. I'm not usually big on female singers, in general, but the four I've just mentioned are amazing, and I'd listen to them any time!
Also concur with everyone who mentioned Freddie Mercury . . . a gorgeous, powerful voice that again, you can't really duplicate.
Agree with Dean about David Bowie - LOVE his voice . . . he can do the offbeat to the sublime with ease.
And I agree with Linda regarding Steve Hogarth . . . would I be too terrible if I said I preferred his voice over Fish? (ducks and runs). And I also love James LaBrie - such a powerful voice that's perfect for the metal edge of DT music.
And speaking of metal, and since we've mentioned Nightwish, I also like Marco Hietala who plays bass, and shares vocal duties with Tarja/Anette (depending on which album you're listening to). He has a rich voice that has that metal screamlike edge to it, without being growly like death metal. I really like his voice. (He's not bad looking either . . .

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And Amy mentioned Ian Anderson and Peter Gabriel . . . my other two "prog holy trinity" artists of the 70's . . . no list would be complete without them.
A couple more no one else has mentioned . . .
I've always adored Greg Lake's voice . . . smooth and rich . . . his rendition of "Still, You Turn Me On", turns ME on . . . wow . . .
And I love Adrian Belew's vocals. Not only is he a guitarist extraordinnaire, but is a passionate singer who can make sense of the quirkiest lyrics . . . VERY cool . . .