I'll go with Ozzy. I think its undeniable Dio is the better vocalist but too me music doesn't get much better than the first four Black Sabbath albums.
Loved Dio with Blackmore's Rainbow. Stargazer one of my favorite under rated album rock songs, also Man on a Silver mountain. Did not care for Dio with Sabbath. That was Ozzie's realm. There was something magic about Ozzie and Iommi at times.