As far as vocal talent goes, Dio wins every time. The guy has such strong vocals, and is one of the most recognisable vocalists you will ever come across.

However, I enjoy Ozzy's music far more. Again, he has a very distinct vocal style. But his lyrics are better, because they are mixture of things, rather than elves and whatnot.

If you go by their times in Sabbath, again Ozzy's music sticks out in my mind more than Dio's. Songs like A National Acrobat, Iron Man, Sweet Leaf etc are all classic rock/metal in my opinion. I always thought the Dio era of Sabbath had more forgettable songs than Ozzy's.

So as far as everything goes, Ozzy takes it for me.