Ozzy, for a couple of reasons.
First, I just like his albums better. They're more consistent; even Ozzy's bad albums I find mostly tolerable. When Dio sucked, he really sucked ... Angry Machines is an abomination.
Beyond that, Ozzy made much more of his talent. He usually had a backing band that was better than him, and on his studio albums he rose to that level. Live he was unreliable, but his studio singing is pretty good. The nasal quality of his voice can get annoying, but he still hits the notes.
After Campbell left, Dio's bands were never a match for Dio's talent. There were some valiant efforts, but it always sounded to me like he got dragged down to the level of his band.
If Dio had made as much from his obviously superior talent as Ozzy made of his, then Dio's death would have been more like Michael Jackson's was.