Ronnie James Dio
Dio is the better singer, but Sabbath's first six albums with Osbourne are superior to the on-off Dio era. Ian Gillan, Ray Gillen and Glenn Hughes are all vastly superior singers to Osbourne, but the material did not match. My favourite Sabbath singer is Tony Martin, although only Tyr stands out. I live in hopes that Iommi and Martin make another album together.
I voted for Dio. If voting between the first Sabbath album and Heaven and Hell, I would vote for the Osbourne album.
I am waiting for possibly Tony Iommi to give the go ahead to the re-mastering and re-releasing of the Tony 'the cat' Martin era Black Sabbath albums especially FORBIDDEN although Tony Martin voice is 'shot' on it. I bought the cd when it was first released back around 1995/6 and thought it was ok but subsequently sold it. The other Tony Martin Black Sabbath albums are HEADLESS CROSS, TYR & CROSS PURPOSES so come on Tony you've got plenty of time on your hands now Sabbath have finished.
E-Z
Yes your 100% correct about The Eternal Idol that's a killer album and a personal favourite of mine as well but the only reason that I didn't include it with the other Tony Martin albums was that it is still available but yes it's a really good listen.You are missing The Eternal Idol, which to me was the best Martin-era Sabbath album, followed closely by Headless Cross.

The Prince of Darkness. Ozzy is arguably one of the most prolific and significant heavy rockers of that genre. Not to mention a damn good lyricist and saaweeet melody writer.