I am surprised LaBrie took such a bashing and didn't get in many peoples' lists.
In my humble opinion, he is a fantastic singer. His control over breathing is almost unmatched and his vibrato and phrasing are fantastic.
Granted there are times when he doesn't sound spot on, but you'll get that with any singer.
His performance when I've seen them play live is incredible, and on all the live recordings, he manages to achieve what he's going for (clean and precise on Scenes From new York, Live At The Marquee and more bluesy and rock on Once In A Livetime and Live In Tokyo).
Some of my other favourites are Daniel Gildenlow. I've seen Pain of Salvation live, and all 3 singers were brilliant. But it's Gildenlow's ability to convey emotion. Simply awesome.
I love Russell Allen's voice. I think his live performance is also fantastic, but his studio work is really inspirational. All his ad lib parts sound fantastic, too.
I thought Layne Stayley had a great voice too, perhaps a bit technically bad, but he was really unique-sounding.