Really i can't decide, Black sabbath is by far my favourite band...i love at the same level both Ozzy era and Dio era albums, i just consider them as two different bands...Dio is awesome ( probably my favourite vocalist after Rob Halford ), but he sucks at Ozzy songs, Ozzy is technicalli not an awesome singer, but no one is able to match him on original sabbath, his voice is unique, especially when it uses his more aggressive and high pitched vocal style, such as in Sabbath bloody sabbath and Symptom of the universe, heavy as hell
Except from technical exstasy and never say die, which sucked.... I'll put Master Of Reality an Paranoid on par at the top of my list...with S.B.S coming close...