Dio is Rainbow, Ozzy is Sabbath. Nuff said.
Truthfully, I didn't care much for the first album. But when I heard the riffs of Electric Funeral, Paranoid became a favorite. Then Master's of Reality, man what an album! And following that up with Supernaut and Snowblind from Vol. 4!!
After that there's a few flashes of greatness... but my all time favorites have to be Vols 2-4.
Now Rainbow had some awesome stuff as well... Catch the Rainbow live is still incredible. They could have done without some of the "live cowd stuff" but still awesome.
Man on the Silver Mountain, Still I'm Sad, Starstruck, Lady of the Lake, etc...
It's one thing we ancients have over you punks... This is the music we grew up with. I remember listening to Black Sabbath, Deep Purple, Rainbow, Judas Priest, Motorhead, AC/DC, Led Zeppelin, Rush, Scorpions, Thin Lizzy, Alice Cooper, Jimi Hendrix...