No flame wars here, man.
The original question was "Who is the best Sabbath vocalist?" I took that to mean, who was the best while actually in Black Sabbath, excluding solo careers, Rainbow, etc. And undoubtedly, the best material was released during Ozzy's era, and no vocalist could have done those 8 albums any better. His voice was perfect for the music. I could name lots of sub-par vocalists, but the musicians around the singer make him that much better. This was the case with Ozzy-era Sabbath.
I dont even consider post-Ozzy Black Sabbath as legit. It was more like Black Sabbath-with-that-dude-from-Rainbow.
As far as vocalists go, Hansi Kursch is more hilarious than anything. How anyone can take that stuff seriously is beyond me. It's so over the top, I can't keep from laughing or cringing. Jag Panzer is the same way.
But yeah, I'll take Sabbath-era Ozzy over those power metal vocalists any day.