Really depends on mood
Dio as a singer,but I like solo Ozzy (up to and including N.r.f.t.w) better.
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."
Dio for sure.
Dio had this particular amazing voice range and he was such a nice man, but Ozzy is more fun. I vote Ozzy.