I wouldn't say that Dehumanizer is a Dio's best with Sabbath but it is the one I listen to the most actually. I absolutely love it. Real raw, no bull metal.
The Devil You Know was an acceptable album too. Nothing amazing but solid. And as has already been mentioned, it was the Mob Rules line-up. They just weren't allowed to use the Black Sabbath name. I think Ozzy has the rights to that now, doesn't he? Or something along those lines anyway.