Sore_Loser is convincing. I don’t know anything about Black Sabbath, had never heard of Dio, and didn’t know Ozzy was in Black Sabbath. That video “Behind the wall of sleep” that Sore_Loser linked to though is much better than anything I can find on youtube that Dio sang. I even like it better than my favorite vocalist, Danny Elfman, which I linked to above. Elfman is a better singer and composer, but I really like the simple sound of “Behind the wall of sleep”.
I bet you never heard this one: http://www.youtube.com/watch?v=yM3ikhjCE_g