I listen to Ozzy's music a lot more often than Dio's, but when I listen to the albums they did just with Sabbath, Ozzy's stuff sounds so primitive by comparison. The Dio material sounds a lot fresher and just more musical. Case in point: Listen to the plodding bass on "War Pigs," then listen to Geezer really PLAY on a song like "Walk Away."
For Black Sabbath, I prefer Ozzy. But Dio did some great work with Sabbath, and I really liked his solo work and with Rainbow. "The Last in Line," "Rainbow in the Dark" and "Holy Diver" are some of my favorite songs.